Active sensors for local planning in mobile robotics 1st Edition Penelope Probert Smith
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- 5. Active sensors for local planning in mobile robotics 1st Edition Penelope Probert Smith Digital Instant Download Author(s): Penelope Probert Smith, Penny Probert Smith ISBN(s): 9789812811141, 9812811141 Edition: 1st File Details: PDF, 22.48 MB Year: 2001 Language: english
- 6. World Scientific Series in Robotics and Intelligent Systems - Vol. 26 ACTIVE SENSORS FOR LOCAL PLANNING IN MOBILE ROBOTICS PENELOPE PROBERT SMITH World Scientific
- 7. ACTIVE SENSORS FOR LOCAL PLANNING IN MOBILE ROBOTICS
- 8. WORLD SCIENTIFIC SERIES IN ROBOTICS AND INTELLIGENT SYSTEMS Editor-in-Charge: C J Harris (University of Southampton) Advisor: T M Husband (University of Salford) Published: Vol. 10: Cellular Robotics and Micro Robotic Systems (T Fukuda and T Ueyama) Vol. 11: Recent Trends in Mobile Robots (Ed. YFZheng) Vol. 12: Intelligent Assembly Systems (Eds. M Lee and J J Rowland) Vol. 13: Sensor Modelling, Design and Data Processing for Autonomous Navigation (M D Adams) Vol. 14: Intelligent Supervisory Control: A Qualitative Bond Graph Reasoning Approach (H Wang and D A Linkens) Vol. 15: Neural Adaptive Control Technology (Eds. R Zbikowski and K J Hunt) Vol. 17: Applications of Neural Adaptive Control Technology (Eds. J Kalkkuhl, KJ Hunt, R Zbikowski and A Dzielinski) Vol. 18: Soft Computing in Systems and Control Technology (Ed. S Tzafestas) Vol. 19: Adaptive Neural Network Control of Robotic Manipulators (SSGe.TH Lee and C J Harris) Vol. 20: Obstacle Avoidance in Multi-Robot Systems: Experiments in Parallel Genetic Algorithms (MAC Gill and A YZomaya) Vol. 21: High-Level Feedback Control with Neural Networks (Eds. F L Lewis and Y H Kim) Vol. 22: Odour Detection by Mobile Robots (R. Andrew Russell) Vol. 23: Fuzzy Logic Control: Advances in Applications (Eds. H B Verbruggen and R Babuska) Vol. 24: Interdisciplinary Approaches to Robot Learning (Eds. J. Demiris and A. Birk) Vol. 25: Wavelets in Soft Computing (M. Thuillard)
- 9. World Scientific Series in Robotics and Intelligent Systems - Vol. 26 ACTIVE SENSORS FOR LOCAL PLANNING MOBILE ROBOTICS PENELOPE PROBERT SMITH University of Oxford, UK V f e World Scientific « • NewJersev London* Sine NewJersey'London • Singapore • Hong Kong
- 10. Published by World Scientific Publishing Co. Pte. Ltd. P O Box 128, Farrer Road, Singapore 912805 USA office: Suite IB, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ACTIVE SENSORS FOR LOCAL PLANNING IN MOBILE ROBOTICS World Scientific Series in Robotics and Intelligent Systems - Volume 26 Copyright © 2001 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in anyform or by any means, electronic or mechanical, includingphotocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN 981-02-4681-1 Printed in Singapore by World Scientific Printers
- 11. Preface The goal of realising a machine which mimics the human ability to refine and structure behaviour in a complex, dynamic world continues to drive mobile robot research. Central to such ability is the need to gather and manipulate rich information on the surroundings. Such a grand ambition places stringent requirements on the sensing systems and on the interaction between sensor and task. One thing which has become clear in attempts to achieve this is the need for diversity in sensing systems. The human vision system remains the in- spiration for artificial analogues, but none can approach its sophistication in terms of hardware or processing. Structured light systems, which mea- sure range directly through using a light source to probe a specific area, are a more reliable method for artificial planning. Their equivalent in sound, sonar, has increased in adaptability and reliability, driven by collaboration with bat biologists as well as from the more standard and established radar literature. Radar itself is becoming cheaper. Given such diversity, another requirement is a structure and method- ology to share and optimise information. Two important paradigms have arisen as a result. One is the idea of the logical sensor which hides the de- tails of the physical sensing operation, so sensors may be specified in terms of task and not in terms of technology: hence a task might require, for example, a sensor to find line segments under particular conditions, rather than a particular technology such as sonar. The other is the active sensor, which abstracts and selects information according to demand - whether this is through probing the environment physically - for example through emit- ting radiation (the traditional active sensor) or through choice or tuning
- 12. VI Preface of algorithms. This concept is an extension of the traditional formulation of the active sensor which interacts with the environment through emit- ting radiation such as sound or light. By developing sensors within this framework we avoid the bottleneck of a large information repository. Much of the work in this book is the result of research with which the editor has been associated in Oxford. It is designed both to provide an overview of the state of the art in active range and vision sensing and to suggest some new developments for future work. It describes real systems and sensors. Cross references have been included between chapters to de- velop and relate concepts across and within a single sensing technique. The book starts with a brief overview of the demands for local planning, discussing the problem of finding a reliable architecture to handle complex- ity and adaptability. It describes the concept of the active sensor, driven by the task in hand and filtering information for that task, to provide a fast, tight sensing-planning loop. It gives an overview of common sensing technologies. In mobile robots, a key requirement for planning is to find out where the robot is within a known region - the localisation problem. Mapping, the problem of extracting geometric or feature based information often un- derlies this. Reliable mapping and localisation requires robust and versatile sensors, and also a systematic method to handle the uncertainty inherent in the sensors and in the robot's own position. Chapter 2 addresses generic issues in mapping and localisation and introduces an important algorithm which is referred to many times in the book, the extended Kalman filter. Sensors which measure range directly are particularly useful for plan- ning. Sensors active in the traditional sense are most important here and most of the book deals with hardware and algorithms for the two most common classes of these: sonar sensors and optoelectronic sensors. The essential factor which distinguishes the way sensors in these classes view the world is their wavelength. Whereas the data from optical sensors naturally falls into standard geometric descriptions such as lines, corners and edges, millimetre wave sensors such as sonar see the world rather dif- ferently. Part II of the book discusses millimetre wave sensors. Significant interpretation is required to extract data for comparison with a standard geometric model. In spite of this, sonar is the commonest sensor used in robotics, largely because of its low cost and easy availability. Another sensor which operates in the millimetre band is high frequency radar - more expen- sive but with very long range and so of great interest outdoors. Although
- 13. Preface vu one of these sensors emits sound waves and the other electromagnetic waves, because of the similar wavelength their data has many similar character- istics. Chapter 3 discusses generally how these characteristics depends on both the sensor geometry (especially the antenna) and target type. Sonar has seen particular developments in the last ten years, from a simple sensor used for obstacle avoidance to a sensor which will produce reliable and robust maps. Chapters 4 to 6 describe how this has been achieved through advances in hardware and data interpretation. Meth- ods of modulation and signal processing drawn from underwater sonar and military radar have been applied to improve resolution and hence extend the range of environments in which sonar operates (chapter 4). Surface modelling, especially the incorporation of rough surface models, has led to better mapping and application in texture recognition (chapter 5). Drawing on analogies from biology, bio-sonar has improved efficiency through sensor placement and small sensor arrays (chapter 6). Finally the application of new processing techniques, especially morphological filtering, has led to the possibility of curve fitting, to produce information which is geometrically similar to our own perception of the world (chapter 7). The problem with sonar is power; the maximum range is limited to around 10m or less (normally closer to 5m). Milimetre wave radar has many similar characteristics but will see over ranges huge by robot stan- dards - over several kilometres depending on weather conditions. For this reason it is of great interest in the field, and the increasing use by the auto- mobile industry (for automatic charging for example) means that the cost is falling, although it is still an expensive technology. Chapter 8 describes the capabilities of radar with a summary of some recent work in robotics. Part III describes sensing at optical wavelengths. Optoelectronic sensors probe the environment using a laser or focussed light emitting diode. At their best, they provide data of high quality which is easy to interpret in terms of standard geometry. However difficulties arise from strong ambient light levels as the active light source can be swamped. A further difficulty in actually realising these systems in the laboratory is the need to scan over one or two dimensions. Unlike scanned sonar, which is compact and light, a scanning optoelectronic sensor imposes power and weight demands which place restrictions on its speed and reactivity. Because of this most applications in local planning gather only two dimensional data (often range versus orientation). Some of these issues are discussed in chapter 9, which also describes some common optical methods to measure range. Chapter
- 14. Vlll Preface 10 describes in detail a sensor based on a technology which has been of particular importance in robotics, amplitude modulated continuous wave (AMCW) operation, often known as lidar. The following chapter (chapter 11) describes the extraction of lines and curves from this and other types of optical range sensor. Chapter 12 describes active vision, in a system which allows the camera to select features of interest and to maintain these in the centre of its field of view through a multi-degree of freedom head. It is impossible to do justice to such an important subject in a book of this scope and it is hoped that this chapter, besides describing a state of the art system for mapping and localisation, will encourage the reader to pursue more specialised texts. The final part of ths book, Part IV, considers some general issues in sensor management. Chapter 13 describes a system which is showing real benefits for processing visual and infra red data. In addition it introduces the more abstract areas of adaptive sensor and knowledge representation. The ultimate goal of autonomy remains elusive, but there are many examples of systems influenced strongly by robotics research. Bumper mounted sonar has been introduced as a parking aid in cars; radar is com- mon not just for speed detection but for automatic charging. Surveillance systems draw on active vision to process and abstract information. The multi-agent paradigms used for routing in Internet access have their coun- terparts in behavioural robotics. The demand for indoor localisation has expanded into areas such as environmental monitoring as a response to the availability of GPS outdoors. The developments described in this book are relevant to all those who are looking for new and improved ways to handle task orientated informa- tion from sensors. It is directed at a final year undergraduate or first year postgraduate level, as well as being of use as a source of ideas to researchers and interested practitioners. Inevitably it has only been able to cover some of the work going on in the field. However I have enjoyed the opportunity to put this book together and I hope that the reader will capture some of the excitement of our research and will use the bibliography as a springboard for their own further investigations. Penelope Probert Smith University of Oxford
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- 16. moss, and little rivers dripping from rock to rock in the stillness, like the sound of falling pearls. But the spell over the wood was above all the spell of beauty, the spell of a breathless enchantment, a spell so deep that the wild-rose growing there seemed other than the wild-rose that grew outside, seemed indeed enchanted, and the very blackberries growing on the great cages of bramble, humming with bees and flickering with butterflies, seemed a magic fruit— which I ate with a beautiful fear that I should be changed into a milk-white fawn, or suddenly find myself a little silver fish in the stream yonder, with the Princess’s lost wedding ring in my inside. The Princess! Why was it that almost from the first I associated the wood with a beautiful princess? I seemed always to be expecting her at some turning of the green pathways, riding upon a white palfrey. Of course, she would be riding upon a white palfrey. Or, perhaps I should come upon her suddenly in one of the sunny openings of the wood, combing her black hair with a golden comb. Or, perhaps she was dead, and this wild-rose was growing up out of her pure wild heart. I made up many stories about her, but this was the story that took strongest hold of my fancy—that she had lost her way in the wood, and at last, worn out with weariness and hunger, had lain her down and died—just here where this rose-bush had drawn its fragrance from her last sweet breath, and its bloom from her fading cheek. I used to sit for hours by the rose-bush, and picture her lying beneath with her eyes closed and a gold crown upon her head, and at morning when the roses were filled with dew, I would say to myself: “O the beautiful Princess! She has been weeping in the night.” And then I would drink her tears out of the little pearl cups of the rose; but I was careful never to mar the tender petals, lest the Princess should feel the pain of it down in the aromatic mould. One day, however, my fancy took another turn, and I said to myself that perchance if I were to pluck one of her roses, the Princess would wake from her enchanted sleep, and stand before me with her strange death-sleepy eyes, and ask me the way back to her lost castle. So one morning when the roses were more than usually drenched with the tears of the Princess, I took heart and plucked the most beautiful rose, saying as I plucked it: “Arise, little Princess and I will take you back to your castle.” Then I waited, and presently I seemed to hear a sigh of happiness, like a spring zephyr, just behind me. I turned, and there stood a maiden with black hair, and eyes the
- 17. colour of which I could not rightly discern, because they seemed filled with moonlight. “Are you the Princess?” I asked. “Yes!” she answered, “I am the Princess, and my name is Once-Upon-a- Time.” “Beautiful Princess,” I said, “may I take you back to your castle?” “Are you sure you know the way, little man?” she said, “for I have been asleep so long that I have quite forgotten it.” “O yes!” I answered eagerly, though really I was far from sure—but I knew that I had friends in the wood on whom I could rely, if by chance I took the wrong turning. So, “O yes!” I answered, “I have in my wanderings passed by your castle many a time. It stands high among the rocks in the middle of the wood, so high among the summer clouds that it makes one dizzy to look up at it, with its donjons and keeps and draw-bridges and battlements, glittering with men-at-arms, and here and there, blowing loose among the stone towers, the bright hair of some beautiful waiting-woman, watching the dark avenues of the woods for the returning huntsmen, and one loved face among the merry horns. All around the castle grow the oldest trees of the wood, very close and dark, and seeming to touch the sky; and thereabout are grim rocks, and hollow caves haunted by dragons and many another evil thing. In one of these a giant lives, so terrible that the bravest knights have gone up against him—only to leave their bones to whiten at the mouth of his cave. And by the castle walls runs an enchanted river, in which live beautiful water-witches, that sing in the moonlight, and draw the lonely home-returning knight down into their watery bowers. In the castle itself is one tower loftier than all the rest, with windows on every side, through which you can see, as in a magic glass, the whole wide earth, with its cities and its roads and all its hidden places. And there, all day long, sits an aged wizard listening to the world, and weaving his spells——” “Yes!” said the Princess, perhaps a little impatient at my long description. “That is my castle. But are you quite sure that you know the way?” At that moment there came and perched upon a bough close by one of those friends, on whom, as I said, I was relying to help me out if I should lose my way. It was a Blue-Bird, with which I had become well-acquainted in my rambles in the wood.
- 18. “Wait a moment, Princess,” I said. “To make quite sure, I will consult this friend of mine here.” Now I must explain that the Blue-Bird, being himself a singer, it is necessary to address him in song. Plain prose he is quite unable to understand. So, if I had said: “Blue-Bird, please tell me the way to the Castle of Princess Once-Upon-a-Time,” he would have shaken his head like a deaf man. Therefore, I spoke to him in this fashion instead; or, rather, I should say that this is the grown-up meaning of what I sang—for the actual song I have forgotten:
- 19. O Blue-Bird, sing the hidden way To Once-Upon-a-Time; We know you cannot speak in prose, So answer us in rhyme. Blue-Bird of Dreams, alone you know The way the dream-folk take, O tell us the right way to go, Before, Blue-Bird, we wake. Dreamers, we seek the way of dreams— O you that know so well Each twist and turning of the way, Blue-Bird, will you not tell? Blue-Bird, if aught that we possess Has any worth to you, O take it, Blue-Bird, here it is, But tell us what to do. The way of dreams, the wonder-way, Wonder and winding streams, Blue-Bird, two dreamers ask of you To point the way of dreams. The way is dangerous, we know, And much beset with dread; But then, it is the only way, Blue-Bird, we care to tread. For this we know: no fact or fear Of the dream-world we seek Can be so terrible to us As those that, week by week, Day in, day out, bleach and benumb The sacred self sincere, The death domestic who hath faced Hath faced the whole of fear
- 20. Hath faced the whole of fear. We are so fearful we may lose The thrill and scent of things, Forget the way to smell a flower, Hear a bird when it sings. O Blue-Bird, sing us on our way Beyond the world that seems— Two dreamers who have lost their way— Back to the world of dreams. To this the Blue-Bird made answer in a song, which, as before, I translate into grown-up language:
- 21. The way of dreams—the Blue-Bird sang— Is never hard to find, So soon as you have really left The grown-up world behind. So soon as you have come to see That what the others call Realities, for such as you, Are never real at all; So soon as you have ceased to care What others say or do, And understand that they are they, And you—thank God!—are you. Then is your foot upon the path, Your journey well begun, And safe the road for you to tread, Moonlight, or morning sun. Pence of this world you shall not take, Yea! no provision heed; A wild-rose gathered in the wood Will buy you all you need. Hungry, the birds shall bring you food, The bees their honey bring; And, thirsty, you the crystal drink Of an immortal spring. For sleep, behold how deep and soft With moss the earth is spread, And all the trees of all the world Shall curtain round your bed. Enchanted journey! that begins Nowhere and nowhere ends, Seeking an ever-changing goal
- 22. Seeking an ever-changing goal, Nowhither winds and wends. For destination yonder flower, For business yonder bird, Aught better worth the travelling to I never saw or heard. O long dream-travel of the soul! First the green earth to tread— And still yon other starry track To travel when you’re dead. With directions so explicit, it was next to impossible to miss the way. So, with little hesitation, Princess Once-Upon-a-Time and I stepped out through the old wood on the way to her castle. As we went along, she told me many things that I have never forgotten, for all of them have come true; but it is necessary for the reader to be reminded that I was still quite a boy, little more than a child, and was, therefore, too inexperienced to give the proper value to what she told me. This speech of hers particularly has remained with me. She said it as we were nearing the end of our walk together, and the turrets of her castle were coming in sight. “This is not the last time we shall meet,” she said, “indeed, we shall meet many times. In a sense we shall be always meeting, though you may not recognise me; for you are one of those who are born my subjects. You are one of those for whom there is no Present, no Future. Your life will always be lived as a dream of What-Might-Have-Been, or What-Once-Was. Your happiness will always be—once-upon-a-time! You are of those who are foredoomed to love the shadow of joy, and the dream of love. Nothing real will ever happen to you—for the reason that your experience will be forever haunted by the more beautiful things that might have happened, or once- upon-a-time did happen to more fortunate men. No beauty will ever seem beautiful enough—for your eyes will be always upon Helen of Troy, or Cleopatra of Egypt. However bright your fortune, the will-o’-the-wisp of a brighter fortune will continually flicker before you. Your dream can never be fulfilled—because it is so entirely a dream. All your days you shall be possessed of old stories, and forgotten fancies, and you shall love only the face you shall never find.”
- 23. And, as she ended, Princess Once-Upon-a-Time bade me farewell, for by this we had come to the gate of her castle. I went back home through the wood, with her eyes in my heart, and her words talking to-and-fro in my brain. Twice I lost my way, but the friends on whom I relied did not forsake me. Once it was a beautiful little snake that zig-zagged in front of me till we came to the right turning. And once it was a chipmunk that seemed to know everything. By the time I came to the home-end of the wood, the stars were rising, and the little creatures of the night were creaking and whirring about me. The windows of home were shining with lamps—welcome beacons, no doubt you will say—and yet, strange as it may sound, I was rather sorry to come upon them so easily. They seemed so safe and comfortable—bed at nine and oatmeal porridge in the morning. I knew that so soon as I lifted the latch all mystery was at an end. Even the punishment that would surely fall upon me for my truancy was quite unmysterious—almost as familiar as my porridge. Bed and porridge—and those voices in the wood! O anti-climax of a wonderful day. How truly had the Princess spoken. What was home to me—with its trimmed lamps, and its quiet carpets and its regular hours; what was home compared with those night-voices and the rising moon. Still, being hungry, I chose the kitchen door, and by a friendly domestic was smuggled away to bed—with a stomach full of pleasant dreams. Such was my first meeting with Once-Upon-a-Time. Next time I met her my boyhood was gone by, and my fancy was no longer occupied with the nursery-stories of which the Blue-Bird had sung. Giants and dragons were receding from my imagination, and my fancy, I must confess, was beginning to take a more sentimental turn. The wood still remained my wonderland, but the wonders I sought there were of a different, if scarcely less dangerous, character. By this I had exchanged my nursery-books for the Mort D’Arthur and Spenser and Shakespeare and such like romantic literature; and my head was, therefore, full of the beautiful ladies and noble lovers of old time. I fear there is no denying that I had by this become quite bookish, and you could scarcely have encountered me in the wood or elsewhere, without some poet or some old playbook under my arm. Ah, how happy were those long summer mornings when I would lie upon a green bank, absorbed in some honeyed tale of lovers dead
- 24. and gone, with the green boughs above sunnily silhouetted on the page. And, just as when a boy the wood had been the scene of all my old nursery- stories, so still it served me as the stage for all my romantic heroes and heroines. It was by turns every wood mentioned in my poets. Of course, it was, first and foremost the Forest of Arden; and one particular glade presided over by a giant oak was easily identified by me as the green courtroom of the banished Duke. As for Jacques, I felt myself his very brother, and replenished the woodland streams with sentimental tears, with no less enjoyment of my own melancholy than he. Rosalind, of course, I was expecting to meet with every moment, and did not fail to inscribe the tree-trunks with sundry rhymes which I hoped might catch her eye. Of these I may have a story to tell later. When the wood was in darker moods, when it wore its tragic mask of thunder and lightning, or put on some sinister witchery of twilight, I would say that Macbeth was on his way to meet the weird sisters. Sometimes, it was “a wood near Athens,” or at others, remembering my Keats, it was that “forest on the shores of Crete,” where Lycius met the snake-woman Lamia. The wood, indeed, was filled with memories of Keats, and if any one in the world knew where the lover of Isabella had been buried by her murderous brothers, surely it was I. I too had discovered the hollow oak where Merlin lay entranced; and many a night, hidden behind the bole of some gigantic beech, had watched Selene bend in a bright crescent above her sleeping shepherd lad. But it is time I told you of my second meeting with Once-Upon-a-Time. I was lying in a bower of wild-roses which I had purposely trained to resemble the bower in which Nicolete slept the night when she fled from the castle of Beaucaire, as we have all read in the delectable history of the loves of Aucassin and Nicolete. It was the golden end of afternoon, and the shadows were still made half of gold. I was lying face down over my book, when suddenly I seemed aware of a new presence near me—as one is conscious that a bird had alighted on a bough close by, or a flower newly opened. Being accustomed to such companions, I did not look up. I was too deep in the loves of my book folk, and too anxious to finish the long euphuistic chapter before the setting sun should warn me of dinner-time. But presently a low laugh sounded behind me, and the sweetest of voices said: “Young sir, you are very selfish with that great book there”—I may say that it was a folio Arcadia of Sir Philip Sidney—“it is so big that I am sure
- 25. that there is room for two pairs of eyes—” “Come read with me,” said I, looking up and blushing. “Nay, I am no Francesca,” she answered; “I would not interrupt your reading, young Paolo.” “But I am tired of reading,” I said, closing the old book. “The sun will soon be gone,” she answered. “Had you not better finish your chapter?” “I would rather finish it by moonlight,” I answered, looking into her eyes. “You are a saucy stripling,” she said. “I should not be surprised if you wrote these lines I just found on yonder tree.” “What lines?” I asked; for the trees, to tell the truth, were tattooed with my verses. “These,” she answered. “O these!” I said, laughing. “Read them to me,” she said. “But they are so long,” I hesitated, “no less than a chant-royal—a Prayer to the Queen of Love, in five long verses, and an envoi! Are you quite sure you can support so much verse at one sitting—” “I have not lived at the Court of King Renée for nothing,” she replied, laughing. “The Court of King Renée!” I exclaimed, looking at her in amazement. “You have really lived there? How wonderful! Tell me about it.” “Indeed, I have!” she answered, with a mocking expression that seemed strangely at variance with her romantic privileges. “O yes! No doubt it is a wonderful place for you ballad-making gentlemen. There you can strum and hum all day to your heart’s content, and your poor bored mistresses must listen to all your magniloquent nonsense, without a yawn—besides being quite sure that you don’t mean a single word of it. Yes! No woman can live at the Court of King Renée unless she is prepared for poetry morning, noon and night—Yes! and far into the middle of the night—and even, when at last you have fallen asleep again, after being awakened by some long- winded serenade, you are barely off, when, with the first break of dawn, comes another fool beneath your window with his lute and his falsetto singing you an ‘aubade!’ An aubade, indeed! And you at last so beautifully
- 26. asleep. As you would have your lady love you, dear youth—never sing her an aubade!” “I marvel that, with such a distaste for song-craft,” I said, “that you should bid me read you a chant-royal, a form so much longer than the aubade——” “O that is different! It is not made use of to wake beautiful ladies from their sleep at unreasonable hours, but reminds one of dreamy old orchards in summer afternoons, and the drowsy bees and the flitting butterflies, and the sea a flickering riband of blue in the distance. It is like the murmur of a beautiful voice talking low to a beautiful lady in the still summer afternoon. The sound of the voice is soothing, and one pays no heed to the words. Besides,” she ended, laughing, “I like the poet, and that makes a great difference——” At this I bent low and kissed her hand, and without further parley began to read:
- 27. O mighty Queen, our Lady of the fire, The light, the music, and the honey, all Blent in one power, one passionate desire Man calleth Love—‘Sweet Love,’ the blessed call— I come a sad-eyed suppliant to thy knee, If thou hast pity, pity grant to me; If thou hast bounty, here a heart I bring For all that bounty thirst and hungering; O Lady, save thy grace, there is no way For me, I know, but lonely sorrowing— Send me a maiden meet for love, I pray! I lay in darkness, face down in the mire, And prayed that darkness might become my pall; The rabble rout roared round me like some quire Of filthy animals primordial; My heart seemed like a toad eternally Prisoned in stone, ugly and sad as he; Sweet sunlight seemed a dream, a mythic thing, And life some beldam’s dotard gossiping: Then Lady, I bethought me of thy sway, And hoped again, rose up this prayer to wing— Send me a maiden meet for love, I pray! Lady, I bear no high resounding lyre To hymn thy glory, and thy foes appal With thunderous splendour of my rhythmic ire; A little lute I lightly touch, and small My skill thereon: yet, Lady, if it be I ever woke ear-winning melody, Twas for thy praise I sought the throbbing string, Thy praise alone—for all my worshipping Is at thy shrine, thou knowest, day by day; Then shall it be in vain my plaint to sing? Send me a maiden meet for love, I pray! Yea! Why of all men should this sorrow dire
- 28. Unto thy servant bitterly befall? For, Lady, thou dost know I ne’er did tire Of thy sweet sacraments and ritual; In morning meadows I have knelt to thee, In noontide woodlands hearkened hushedly Thy heart’s warm beat in sacred slumbering, And in the spaces of the night heart ring Thy voice in answer to the spheral lay: Nowneath thy throne my suppliant life I fling— Send me a maiden meet for love, I pray! I ask no maid for all men to admire, Mere body’s beauty bath in me no thrall, And noble birth, and sumptuous attire, Are gauds I crave not—yet shall have withal, With a sweet difference, in my heart’s own She, Whom words speak not, but eyes know when they see, Beauty beyond all glass’s mirroring, And dream and glory hers for garmenting; Her birth—O Lady, wilt thou say me nay?— Of thine own womb, of thine own nurturing— Send me a maiden meet for love, I pray! ENVOI Sweet Queen who sittest at the heart of spring, My life is thine, barren or blossoming; Tis thine to flush it gold or leave it grey: And so unto thy garment’s hem I cling— Send me a maiden meet for love, I pray. “I wonder,” I said after a little while, when she had praised my verses, and I sat by her side holding her hands and looking into her strange far- away eyes, “I wonder if you are the answer to my prayer—for so soon as I looked upon you, I gave you all my love, and, if you cannot give me yours in return, my heart will break—” She shook her head sadly, and her eyes seemed to grow still more far- away, but she made no answer more, for all my entreaties, till at last the day
- 29. had gone, and the moon was rising through the wood—and she still sitting by my side like a spirit in the spectral light. Once I seemed to hear her moan in the silence, and a shiver passed through her body. Then she turned her eyes upon me—they seemed like wells brimming with stars: “I love you,” she said, “but we can never be each other’s. My name is Once-Upon-a-Time.” At this I threw myself at her feet face down in the grass and wept bitterly, and I felt her hand soothingly laid upon my hair, and heard her voice softly bidding me be comforted. And for a long time it was so with us, till methinks I must have fallen asleep of the sweet soothing of her hand on my hair, and the murmur of her sweet voice—for, when I raised my head from the grass, the place was empty and the dawn was stealing with feet of pearl through the wood. The dawn! “She feared,” I cried, bitterly, “she feared that I might sing her my aubade!” But this, of course, was only the lip-cynicism of my sad young heart, stricken with the arrows of that haunted beauty. Once-Upon-a-Time! Thus had the Princess met me again as she had said, and often as I grew up to be a man, and walked but seldom in that old wood of dreams, her words would come back to me: “You are of those who are foredoomed to love the shadow of joy, and the dream of love. Your happiness will always be—Once-Upon-a Time.” For, as I walked the ways of the world, I saw that my old wood had only been a dream picture of the real world outside, and that the real world itself, in which my manhood was now called on to play its part, was no less a dream of beauty and terror, of love and death, of good and evil, than my old wood itself; and, like my old wood, it seemed haunted for me by the face of a Princess—some dear, desired face of woman lost amid these drifting faces, as in my boyhood it had been lost among the leaves of the wood. Beautiful faces, beautiful faces, drifting by in the crowded streets—but never my face among all the faces. Hints of my face, even glimpses perhaps—sometimes almost the certainty that it is she yonder—but a sudden turn of the head, and alas! It is not she! Yet a day did come at last, when the mob of unmeaning faces seemed suddenly to open, as the clouds fall away right and left before the
- 30. moon; or as in a wilderness of leaves without a blossom, one should come upon the breathless beauty of some lonely flower. Yes! It was my face at last. We looked at each other but for a moment in the street, which her beauty had suddenly made silent for me as the desert—but for a moment, yet Eternity must be like that look we gave each other. Then, though she spoke no audible word, my heart heard her say: “Look in my face; my name is Might-Have-Been; I am also called No- More, Too-Late, Farewell.” On one of her beautiful fingers my sad eyes had caught the glimmer of a small gold band—and, once more as we passed away from each other, my bitter heart mocked at its own bitterness, and remembering my boyish fairy- tales, I said to myself: “The Princess has found her wedding ring!” And that was my last meeting with Princess Once-Upon-a-Time.
- 31. M THE LITTLE JOYS OF MARGARET ARGARET had seen her five sisters one by one leave the family nest to set up little nests of their own. Her brother, the eldest child of a family of seven, had left the old home almost beyond memory and settled in London. Now and again he made a flying visit to the small provincial town of his birth, and sometimes he sent two little daughters to represent him—for he was already a widowed man and relied occasionally on the old roof-tree to replace the lost mother. Margaret had seen what sympathetic spectators called her “fate” slowly approaching for some time —particularly when, five years ago, she had broken off her engagement with a worthless boy. She had loved him deeply, and, had she loved him less, a refined girl in the provinces does not find it easy to replace a discarded suitor—for the choice of young men is not excessive. Her sisters had been more fortunate, and so, as I have said, one by one they left their father’s door in bridal veils. But Margaret stayed on, and at length, as had been foreseen, became the sole nurse of a beautiful old invalid mother, a kind of lay sister in the nunnery of home. She came of a beautiful family. In all the big family of seven there was not one without some kind of good looks. Two of her sisters were acknowledged beauties, and there were those who considered Margaret the most beautiful of all. It was all the harder, such sympathizers said, that her youth should thus fade over an invalid’s couch, the bloom of her complexion be rubbed out by arduous vigils, and the lines prematurely etched in her skin by the strain of a self-denial proper, no doubt, to homely girls and professional nurses, but peculiarly wanton and wasteful in the case of a girl so beautiful as Margaret. There are, alas! a considerable number of women predestined by their lack of personal attractiveness for the humbler tasks of life. Instinctively we associate them with household work, nursing, and the general drudgery of existence. One never dreams of their having a life of their own. They have no accomplishments, nor any of the feminine charms. Women to whom an offer of marriage would seem as terrifying as a comet, they belong to the neutrals of the human hive, and are, practically speaking, only a little higher
- 32. than the paid domestic. Indeed, perhaps, their one distinction is that they receive no wages. Now for so attractive a girl as Margaret to be merged in so dreary, undistinguished, a class was manifestly preposterous. It was a stupid misapplication of human material. A plainer face and a more homespun fibre would have served the purpose equally well. Margaret was by no means so much a saint of self-sacrifice as not to have realised her situation, with natural human pangs. Youth only comes once—especially to a woman; and No hand can gather up the withered fallen petals of the Rose of youth. Petal by petal, Margaret had watched the rose of her youth fading and falling. More than all her sisters, she was endowed with a zest for existence. Her superb physical constitution cried out for the joy of life. She was made to be a great lover, a great mother; and to her, more than most, the sunshine falling in muffled beams through the lattices of her mother’s sick-room came with a maddening summons to—live. She was so supremely fitted to play a triumphant part in the world outside there, so gay of heart, so victoriously vital. At first, therefore, the renunciation, accepted on the surface with so kind a face, was a source of secret bitterness and hidden tears. But time, with its mercy of compensation, had worked for her one of its many mysterious transmutations, and shown her of what fine gold her apparently leaden days were made. She was now thirty-three; though, for all her nursing vigils, she did not look more than twenty-nine, and was now more than resigned to the loss of the peculiar opportunities of youth—if, indeed, they could be said to be lost already. “An old maid,” she would say, “who has cheerfully made up her mind to be an old maid, is one of the happiest, and, indeed, most enviable, people in all the world.” Resent the law as we may, it is none the less true that renunciation brings with it a mysterious initiation, a finer insight. Its discipline would seem to refine and temper our organs of spiritual perception, and thus make up for the commoner experience lost by a rarer experience gained. By dedicating herself to her sick mother, Margaret undoubtedly lost much of the average experience of her sex and age, but almost imperceptibly it had been borne in upon her that she made some important gains of a finer kind. She had
- 33. been brought very close to the mystery of human life, closer than those who have nothing to do beyond being thoughtlessly happy can ever come. The nurse and the priest are initiates of the same knowledge. Each alike is a sentinel on the mysterious frontier between this world and the next. The nearer we approach that frontier, the more we understand, not only of that world on the other side, but of the world on this. It is only when death throws its shadow over the page of life that we realise the full significance of what we are reading. Thus, by her mother’s bedside, Margaret was learning to read the page of life under the illuminating shadow of death. But, apart from any such mystical compensation, Margaret’s great reward was that she knew her beautiful old mother better than any one else in the world knew her. As a rule, and particularly in a large family, parents remain half mythical to their children, awe-inspiring presences in the home, colossal figures of antiquity, about whose knees the younger generation crawls and gropes, but whose heads are hidden in the mists of pre-historic legend. They are like personages in the Bible. They impress our imagination, but we cannot think of them as being quite real. Their histories smack of legend. And this, of course, is natural; for they had been in the world, had loved and suffered, so long before us that they seem a part of that ante-natal mystery out of which we sprang. When they speak of their old love-stories, it is as though we were reading Homer. It sounds so long ago. We are surprised at the vividness with which they recall happenings and personalities past and gone before, as they tell us, we were born. Before we were born! Yes! They belong to that mysterious epoch of time—“before we were born”; and unless we have a taste for history, or are drawn close to them by some sympathetic human exigency, as Margaret had been drawn to her mother, we are too apt, in the stress of making our own, to regard the history of our parents as dry-as-dust. As the old mother sits there so quiet in her corner, her body worn to a silver thread, and hardly anything left of her but her indomitable eyes; it is hard, at least for a young thing of nineteen, all aflush and aflurry with her new party gown, to realise that that old mother is infinitely more romantic than herself. She has sat there so long, perhaps, as to have come to seem part of the inanimate furniture of home, rather than a living being. Well! the young thing goes to her party, and dances with some callow youth who pays her clumsy compliments, and Margaret remains at home with the old mother in her corner. It is hard on Margaret! Yes; and yet, as I have said, it
- 34. is thus she comes to know her old mother better than any one else knows her—society perhaps not so poor an exchange for that of smart, immature young men of one’s own age. As the door closes behind the important rustle of youthful laces, and Margaret and her mother are left alone, the mother’s old eyes light up with an almost mischievous smile. If age seems humorous to youth, youth is even more humorous to age. “It is evidently a great occasion, Peg,” the old voice says, with the suspicion of a gentle mockery. “Don’t you wish you were going?” “You naughty old mother!” answers Margaret, going over and kissing her. The two understand each other. “Well, shall we go on with our book?” says the mother, after a while. “Yes, dear, in a moment. I have first to get you your diet, and then we can begin.” “Bother the diet!” says the courageous old lady; “for two pins I’d go to the ball myself. That old taffeta silk of mine is old enough to be in fashion again. What do you say, Peg, if you and I go to the ball together?” “O it’s too much trouble dressing, mother. What do you think?” “Well, I suppose it is,” answers the mother. “Besides, I want to hear what happens next to those two beautiful young people in our book. So be quick with my old diet, and come and read.” There is perhaps nothing so lovely, or so well worth having, as the gratitude of the old towards the young that care to give them more than the perfunctory ministrations to which they have long since grown sadly accustomed. There was no reward in the world that Margaret would have exchanged for the sweet looks of her old mother, who, being no merely selfish invalid, knew the value and the cost of the devotion her daughter was giving her. “I can give you so little, my child, for all you are giving me,” her mother would sometimes say; and the tears would spring to Margaret’s eyes. Yes! Margaret had her reward in this alone—that she had cared to decipher the lined old document of her mother’s face. Her other sisters had passed it by more or less impatiently. It was like some ancient manuscript in a museum, which only a loving and patient scholar takes the trouble to read.
- 35. But the moment you begin to pick out the words, how its crabbed text blossoms with beautiful meanings and fascinating messages! It is as though you threw a dried rose into some magic water, and saw it unfold and take on bloom and fill with perfume, and bring back the nightingale that sang to it so many years ago. So Margaret loved her mother’s old face, and learned to know the meaning of every line on it. Privileged to see that old face in all its private moments of feeling, under the transient revivification of deathless memories, she was able, so to say, to reconstruct its perished beauty and realise the romance of which it was once the alluring candle. For her mother had been a very great beauty, and if, like Margaret, you are able to see it, there is no history so fascinating as the bygone love-affairs of old people. How much more fascinating to read one’s mother’s love-letters than one’s own! Even in the history of the heart recent events have a certain crudity, and love itself seems the more romantic for having lain in lavender for fifty years. A certain style, a certain distinction, beyond question go with antiquity, and to spend your days with a refined old mother is no less an education in style and distinction than to spend them in the air of old cities, under the shadow of august architecture, and in the sunset of classic paintings. The longer Margaret lived with her old mother, the less she valued the so-called “opportunities” she had missed. Coming out of her mother’s world of memories, there seemed something small, even common, about the younger generation to which she belonged—something lacking in significance and dignity. For example, it had been her dream, as it is the dream of every true woman, to be a mother herself: and yet, somehow—though she would not admit it in so many words—when her young married sisters came with their babies, there was something about their bustling and complacent domesticity that seemed to make maternity bourgeois. She had not dreamed of being a mother like that. She was convinced that her old mother had never been a mother like that. “They seem more like wet-nurses than mothers,” she said to herself, with her wicked wit. Was there, she asked herself, something in realisation that inevitably lost you the dream? Was to incarnate an ideal to materialise it? Did the finer spirit of love necessarily evaporate like some volatile essence with
- 36. marriage? Was it better to remain an idealistic spectator such as she—than to run the risks of realisation? She was far too beautiful, and had declined too many offers of commonplace marriage, for such questioning to seem the philosophy of disappointment. Indeed, the more she realised her own situation, the more she came to regard what others considered her sacrifice to her mother as a safeguard against the risk of a mediocre domesticity. Indeed, she began to feel a certain pride, as of a priestess, in the conservation of the dignity of her nature. It is better to be a vestal virgin than—some mothers. And, after all, the maternal instinct of her nature found an ideal outlet in her brother’s children—the two little motherless girls, who came every year to spend their holidays with their grandmother and their aunt Margaret. Margaret had seen but little of their mother, but her occasional glimpses of her had left her with a haloed image of a delicate, spiritual face that grew more and more Madonna-like with memory. The nimbus of the Divine Mother, as she herself had dreamed of her, had seemed indeed to illumine that grave young face. It pleased her imagination to take the place of that phantom mother, herself—a phantom mother. And who knows but that such dream-children, as she called those two little girls, were more satisfactory in the end than real children? They represented, so to say, the poetry of children. Had Margaret been a real mother, there would have been the prose of children as well. But here, as in so much else, Margaret’s seclusion from the responsible activities of the outside world enabled her to gather the fine flower of existence without losing the sense of it in the cares of its cultivation. I think that she comprehended the wonder and joy of children more than if she had been a real mother. Seclusion and renunciation are great sharpeners and refiners of the sense of joy, chiefly because they encourage the habit of attentiveness. “Our excitements are very tiny,” once said the old mother to Margaret, “therefore we make the most of them.” “I don’t agree with you, mother,” Margaret had answered. “I think it is theirs that are tiny—trivial indeed, and ours that are great. People in the world lose the values of life by having too much choice; too much choice— of things not worth having. This makes them miss the real things—just as any one living in a city cannot see the stars for the electric lights. But we,
- 37. sitting quiet in our corner, have time to watch and listen when the others must hurry by. We have time, for instance, to watch that sunset yonder, whereas some of our worldly friends would be busy dressing to go out to a bad play. We can sit here and listen to that bird singing his vespers as long as he will sing—and personally I wouldn’t exchange him for a prima donna. Far from being poor in excitements, I think we have quite as many as are good for us, and those we have are very beautiful and real.” “You are a brave child,” answered her mother. “Come and kiss me,” and she took the beautiful gold head into her hands and kissed her daughter with her sweet old mouth, so lost among wrinkles that it was sometimes hard to find it. “But am I not right, mother?” said Margaret. “Yes! you are right, dear, but you seem too young to know such wisdom.” “I have to thank you for it, darling,” answered Margaret, bending down and kissing her mother’s beautiful grey hair. “Ah! little one,” replied the mother, “it is well to be wise, but it is good to be foolish when we are young—and I fear I have robbed you of your foolishness.” “I shall believe you have if you talk like that,” retorted Margaret, laughingly taking her mother into her arms and gently shaking her, as she sometimes did when the old lady was supposed to have been “naughty.” So for Margaret and her mother the days pass, and at first, as we have said, it may seem a dull life, and even a hard one, for Margaret. But she herself has long ceased to think so, and she dreads the inevitable moment when the divine friendship between her and her old mother must come to an end. She knows, of course, that it must come, and that the day cannot be far off when the weary old limbs will refuse to make the tiny journeys from bedroom to rocking-chair which have long been all that has been demanded of them; when the brave, humorous old eyes will be so weary that they cannot keep open any more in this world. The thought is one that is insupportably lonely, and sometimes she looks at the invalid-chair, at the cup and saucer in which she serves her mother’s simple food, at the medicine-bottle and the measuring-glass, at the knitted shawl which protects the frail old form against draughts, and at all such sad furniture of
- 38. an invalid’s life, and pictures the day when the homely, affectionate use of all these things will be gone forever; for so poignant is humanity that it sanctifies with endearing associations even objects in themselves so painful and prosaic. And it seems to Margaret that when that day comes, it would be most natural for her to go on the same journey with her mother—and still be her loving nurse in Paradise! For who shall fill for her her mother’s place on earth—and what occupation will be left for Margaret when her “beautiful old raison d’être,” as she sometimes calls her mother, has entered into the sleep of the blessed? She seldom thinks of that, for the thought is too lonely, and, meanwhile, she uses all her love and care to make this earth so attractive and cosey that the beautiful mother-spirit, who has been so long prepared for her short journey to heaven, may be tempted to linger here yet a little while longer. These ministrations, which began as a kind of renunciation, have now turned into an unselfish selfishness. Margaret began by feeling herself necessary to her mother; now her mother becomes more and more necessary to Margaret. Sometimes when she leaves her alone for a few moments in her chair, she laughingly bends over and says, “Promise me that you won’t run away to heaven while my back is turned.” And the old mother smiles one of those transfigured smiles which seem only to light up the faces of those that are already half over the border of the spiritual world. Winter is, of course, Margaret’s time of chief anxiety, and then her efforts are redoubled to detain her beloved spirit in an inclement world. Each winter passed in safety seems a personal victory over death. How anxiously she watches for the first sign of the returning spring, how eagerly she brings the news of early blade and bud, and, with the first violet, she feels that the danger is over for another year. When the spring is so afire that she is able to fill her mother’s lap with a fragrant heap of crocus and daffodil, she dares at last to laugh and say: “Now confess, mother, that you won’t find sweeter flowers even in heaven.” And when the thrush is on the apple bough outside the window, Margaret will sometimes employ the same gentle raillery.
- 39. “Do you think, mother,” she will say, “that an angel could sing sweeter than that thrush?” “You seem very sure, Margaret, that I am going to heaven,” the old mother will sometimes say, with one of her arch old smiles; “but do you know that I stole two peppermints yesterday?” “You did!” says Margaret. “I did indeed!” answers the mother, “and they have been on my conscience ever since.” “Really, mother! I don’t know what to say,” answers Margaret. “I had no idea that you are so wicked.” Many such little games the two play together, as the days go by; and often at bedtime, as Margaret tucks her mother into bed, she asks her: “Are you comfortable, dear? Do you really think you would be much more comfortable in heaven?” Or sometimes she will draw aside the window-curtains and say: “Look at the stars, mother.... Don’t you think we get the best view of them down here?” So it is that Margaret persuades her mother to delay her journey a little while.
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