Popular Science Monthly/Volume 21/May 1882/The Stereoscope: Its History I
By W. LE CONTE STEVENS.
THAT a near object of small dimensions presents an aspect slightly different to each one of a pair of eyes directed upon it, has been known for more than two thousand years; but no application of this knowledge was ever made until some time after the beginning of the present century. The analysis of binocular vision is one of the products of modern investigation, and the stereoscope is its direct outcome. That vision with two eyes is greatly preferable to what the ancients accorded to Polyphemus is fully appreciated by every one who possesses a pair of healthy visual organs and a stereoscope, but who at any time has been so unfortunate as to suffer a temporary injury that reduces him for a few days to the condition of the classic monocular giant. Familiar as he may be with the truth that the perspective effect of a fine painting is better appreciated when one eye is closed, he is never willing to keep it thus inactive any longer than necessary; and, if such a hint is gently suggested, he is prompt to answer it by some prosaic contrast between the artist's clever illusion and the necessities of life in a wide-awake world. Lord Bacon says: "We see more exquisitely with one eye than with both, because the vital spirits thus unite themselves the more, and become the stronger; for we may find by looking in a glass whilst we shut one eye that the pupil of the other dilates." But even the cogent logic of Lord Bacon would scarcely reconcile many of us to the adoption of strictly Cyclopean customs in the enjoyment of vision.
In response to the question, "What is the use of having two eyes?" the answer has been given, "To have one left if the other is hurt." Much as we may admire the sagacious foresight of this youthful physiologist, it will not be found sufficient to rest contented with his ultimatum. He had evidently not tried his skill to find how unexpectedly he would miss the inkstand while endeavoring to dip his pen into it at arm's length, with one eye closed. He had not thought of holding his finger a few inches in front of his face to find what part of the wall it would hide from each eye in succession, or how differently it would look when regarded from those two points of view separately, how much thicker it would appear when both eyes were open, how readily he could examine three sides of it at once, how much more definitely he could judge its distance, in a word how much more comprehensive was the information given by two eyes if used at the same moment. Assuming that he knows exactly how to account for the inversion of the retinal image and the erect appearance of the object there pictured, how our visual perceptions are only signs of what we momentarily feel on the retina, signs that generally represent the realities with a fair degree of accuracy, but may sometimes represent almost anything else on demand, how, if the eyes be healthy, we have no consciousness of possessing any retina at all, but instantly and unconsciously refer every retinal sensation to some external body whose existence we are obliged to assume, unless there be special arguments to convince us to the contrary—granting all this, our young physiologist has not thought of inquiring how it is that, although two retinal images are produced, we see but a single object, and this despite the fact that, like photographs of the same body simultaneously taken from different stand-points, these two images are necessarily dissimilar.
This question, and especially its latter part, is much more easily asked than answered with fullness, clearness, and certainty. There is no antecedent reason why two separate retinal images should not produce the impression of two separate bodies. That they may do so must have come within the experience of every one. A few glasses of champagne are often enough to convince the most skeptical. Without resorting, however, to agencies that produce involuntary though temporary loss of muscular control of the eyes, it is only necessary to gaze at any clearly defined object for a few moments, and press upon the eyeball near the outer corner of the opening between the lids—double vision is instantly attained. The condition thus induced is unnatural, and the effect is unnatural vision. Our modes of interpreting nerve-impressions, like our modes of mental and bodily action in other respects, are the results partly of individual experience and partly of inheritance through countless generations. If a blow is received upon the right cheek, whether it be from a solid body or from a wave in the medium in which we are immersed, experience at once suggests the direction from which it came. If through many generations every individual were continually receiving gentle blows on the center of the right cheek through all the moments of waking existence, and the accurate perception of these were conducive to his welfare, then on physiological principles it seems in the highest degree probable that the judgment of direction by the cheek might become as habitual and unerring as is our judgment of direction by the eye. By a liberal construction of language we might be said to that extent to see with the cheek; and the man who is blessed with the healthiest, best-trained,
Fig. 1.—Left and Right Projections of Glass Cone.
and most vigorous cheek, aside from other qualities, would be most apt to win in the struggle for existence. If, then, in normal vision each of two eyes receives impressions due to wave-impulses from the same external object, the position of this is referred to the same external place in accordance with the association that experience, individual and ancestral, instantly arouses in response to the sensation felt on each separate retina. If, therefore, every person were blessed with such optic luxuriance as is attributed to the fabled Argus, there is no reason to suppose that, if each eye be healthy and ordinarily controllable, he would have anything else than single vision, unless the necessities imposed in the struggle for success made it advantageous to acquire the power of dissociating the action of the muscles of these eyes, and thus making use of voluntary multiple vision.
In an essay published many years ago, Carlyle dwelt, in a manner characteristically his own, upon the unconsciousness that is a mark of health in the human body. The dyspeptic man knows full well that he has a stomach, but the eupeptic child has no conception of the existence of such an organ, however vivid may be its ideas of fairies, ogres, and dragons. In like manner the retina is an abstraction for him who has good binocular vision and but little book-lore. With a single eye he sees many objects at the same time, and judges their different positions; the only idea aroused is about the objects themselves, and not about the retinal impressions from them. If both eyes be directed to the same distant point, there is still the same consciousness of a single external thing, and not of two eyes. By slowly crossing the two visual lines for the purpose of comprehensively scanning the root of one's own nose, which is the nearest object that can be regarded with entire convenience, if both eyes are of equal power, the visual impression is found to be that two noses are approaching each other, and closing up the brightest part of the field of view in front. Between them is left a narrow heart-shaped window, with dimly transparent nasal shutters. The outlines of these are most easily discerned by momentarily closing each eye alternately, while the convergence of visual lines is vigorously retained, and then opening both and depending on indirect vision. If there is any consciousness of an eye at all, it is referred to the sensation of strain in the muscles that seem to be pulling the shutters together, and not to any retina receiving pictures of them. There is, indeed, the consciousness of looking out of the window from a single stand-point, but not from two eyes. The subjective impression is that the two points of view are identified into a single eye, whose position is central and constitutes the point of origin from which all our estimates of direction and distance are made. Keeping the nasal window as small as possible by cross-vision, and endeavoring to test the real singleness of the double-phantom nose by gently putting the finger upon it from in front, it is easy additionally to convince one's self that
"... things are not what they seem."
Two fingers will be seen approaching from different directions. If it should occur to the indignant observer that these may be utilized in putting an end to his nasal redundancy by closing up the window, they will steadily converge and strike together upon the root of the nose, almost exactly where he had been supposing his point of view to be. The window at the next moment, instead of being closed, will be opened wide, and, on resting the tired muscles of his eyes, he will find that the phantom-noses have leaped to the two sides, the position of each being indicated by the faithful ghosts of the finger. The experiment is a little surprising at first, and the specters are very shadowy, but a literally close search will be quite sure to reveal them by indirect vision.
Subjectively, therefore, our condition is not so very different from that of the famous Cyclops. We have the advantage of being able to see double, by adjusting conditions properly; but, if sensation is to be trusted, the object is duplicated while the eye is single, although by other means we learn that the object remains single, and is only viewed from two different stand-points at the same moment, while the separate
Fig. 2.—Wheatstone's Stereoscope. (Front View.)
Fig. 3.—Wheatstone's Stereoscope. (Ground Plan.)
lines of direction for the two eyes meet elsewhere. By appropriate muscular training the eyes may be directed, each slightly outward, so that these lines meet behind the observer's head while the object, apparently duplicated, is seen still in front. The recognition of the subjective fusion of the two eyes into a Cyclopean, or central binocular eye, is a fundamental prerequisite for the explanation of vision in the stereoscope. In consequence of this, if two similar pictures are placed close in front of the eyes, the distance between their centers being equal to the distance between the pupils, they at once appear to coalesce into a single picture. In this way an objective existence may appear to be given to the binocular eye by approaching a mirror until the nose touches the glass, and avoiding the convergence of visual lines that would otherwise be natural. A narrow face is seen, possessing but a single eye, that looks into the very depths of the observer's Cyclopean eye.
The conception of this subjective union as the product of the experience of the race in interpreting sensations, and the consequent necessity of distinguishing between realities and their visual representations, seems never to have been appreciated until long after the invention of instruments for use in the analysis of vision. Much confusion has resulted from the attempt to explain what are really subjective results of retinal sensation by the application of geometric principles, irrespective of the illusive union of the two eyes when employed together. In 1604 Kepler stated that the distance between the eyes constituted a baseline, which we employ for measuring the distance of objects by a species of visual triangulation. This idea was subsequently greatly elaborated by Sir David Brewster and others; and in most, if not all, of our textbooks of physics today it is applied in a very familiar diagram to explain the principle of the stereoscope. On this theory the apparent position of every point in the stereoscopic field of view is determined by the meeting of separate visual lines, which converge in front. An obvious consequence is that this localization should become impossible if the visual lines become parallel or divergent. But, in truth, there can be no perception of locality by this method. If the eyes are subjectively united, the visual lines become subjectively united along with them; if, indeed, such language is at all applicable to lines that are mere abstractions. In its application to stereoscopic vision, therefore, the diagram is worthless; for such vision is much easier to most persons when the visual lines are parallel, or very slightly divergent, than when they are strongly convergent, and in no case can there be any recognition of intersection between lines which, if subjectively perceived at all, would be coincident throughout their whole extent.
The error just mentioned has undoubtedly sprung from the assumption that stereoscopic vision is always perfectly normal. If this be so, it should be as painless as the reading of this page, even when continued for hours in succession. Every one who has tried the experiment with an ordinary stereoscope, and a large, miscellaneous collection of stereographs, knows how wearying it is, and how in some cases distinct vision is found impossible. To indicate the real differences between normal vision and that which is attained in most stereoscopes, it will be necessary first to study the development of this instrument.
The duality of human vision of near objects, and the consequent dissimilarity of retinal pictures in the separate eyes, was apprehended and more or less vaguely discussed by Euclid (b. c. 300), Galen (a. d. 200), Baptista Porta (1593), Leonardo da Vinci (1584), Aguilonius (1613), and by Smith, Harris, and Porterfield during the eighteenth century. No practical results were wrought, however, until 1838, when Sir Charles Wheatstone read before the Royal Society his now classic paper on the "Physiology of Vision." Let the reader imagine, or actually put on the page before him, some small solid body, such as a cone, with a few lines drawn from its vertex to the base. If it be of glass, so much the better; an ink-dot can then be marked at the center of the base, and the lines scratched upon the sides can easily be blackened. Close the left eye; the cone appears to the right eye like Fig. 1, R. Without moving the head, look with the left eye alone; the appearance is like Fig. 1, L. If each eye were in succession transformed for a moment into an electric light, the shadows projected upon the paper would be those given in the figure, but with a common base. Opening both eyes, the perception of the height of the cone is far more distinct than when either is closed. Let us now quote Wheatstone's own words: "It being thus established that the mind perceives an object of three dimensions by means of the two dissimilar pictures projected by it on the two retinæ, the following question occurs: What would be the visual effect of simultaneously presenting to each eye, instead of the object itself, its projection on a plane surface as it appears to that eye? To pursue this inquiry, it is necessary that means should be contrived to make the two pictures, which must necessarily occupy different places, fall on similar parts of both retinæ. Under the ordinary circumstances of vision, the object is seen at the concourse of the optic axes (visual lines), and its images consequently are projected on similar parts of the two retina?; but it is also evident that two exactly similar objects may be made to fall on similar parts of the two retinæ, if they are placed one in the direction of each optic axis, at equal distances before or beyond their intersection."
Fig. 4.—Wheatstone's Stereoscope (Perspective View), 1833.
To follow out to the letter the instructions suggested in Wheatstone's last sentence, transfer Fig. 1 to glass. This can be easily done. Upon an oblong plate of window-glass put a few drops of clear varnish; let it spread thinly over the surface and become thoroughly dry. Copy the picture, of exact size, by scratching through the varnish, and then blacken the lines with ink. Hold the transparent plate at the distance of a foot from your eyes, and through it look at a point about five feet away. Very little motion of the plate is needed to get this point exactly aligned with each of the dots within the circles by looking with each eye in succession. Look at the point now with both eyes, and you will see, suspended in the air, probably just beyond the plate, apparently a solid cone of glass pointing toward you, the very fac-simile of our glass cone from which the pictures were taken.
Copy the picture also on paper or card-board, of exact size, but with the part marked R transferred to the left, and that marked L to the right. Hold up the point of a pencil about half-way between your eyes and the card. In a moment the proper position is found, where it is aligned with R for the right eye and with L for the left. Open both eyes and converge them upon the pencil-point. A little cone pointing toward you is suspended in the air just beyond the pencil, which may now be withdrawn. Move your head from side to side: the cone moves with you. It is brilliantly lustrous, sharp in outline, and much smaller than that previously seen. Two companion circles, one on each side, are left behind on the card, and are larger than the base of the suspended cone, but a little smaller than the circles originally were. Their appearance is due to images of R and L which fall upon retinal parts that in normal vision could not be simultaneously impressed by
Fig. 5.—The First Landscape Stereograph.
an external single body. The sensations produced by them are hence not suggestive of singleness, and each is therefore referred separately outward in the direction from which the rays producing them have come. Such side-images are perceived also when the glass plate is employed. Try the same experiment now with the picture on the page; the miniature cone leaps off the paper into the air, but this time it is hollow, for its vertex is pointed to the place from which it seems to have sprung.
These experiments are delightfully surprising when successfully accomplished for the first time. They are well worth the trifling preliminary trouble which they entail. But even this can be in great measure avoided by having a photograph of the picture taken on glass. If you will previously approach the polite photographer in your most charmingly courteous and irresistible style, enable him to perceive the glittering phantom cone reversed in mid-air, invite him to grasp it and give to this "airy nothing a local habitation and a name," and convince him that, if not illusive, it is even more elusive than the merry sunbeam which his camera alone can catch in all its beauty, he will at once be lost in admiration of your magic skill and singular sagacity, and instantly find it impossible to avoid preparing the wonder-working photograph on glass. This he will smilingly present to you in the most enthusiastic and complimentary manner, with evident gratitude for the favor you have bestowed, and the good taste you have exhibited in selecting him as the recipient of your discriminating and exclusive confidence.
The presence of the uncombined images at the sides of the binocular picture, as it stands out in solid relief, is apt to be confusing, because their effect is partially to distract the attention. In Wheatstone's first experiments, he avoided them by looking through tubes, or into a box. In any case, the methods of stereoscopy just described, although by far the most useful in studying the principles of binocular vision, are not usually acquired until after a few trials. When they are once mastered, it becomes easy to discard pencils and other points of fixation, and the voluntary muscular control of the eyes is sufficient for all cases. Wheatstone gave to the world a new revelation in both the science and the art of perspective, when, in 1838, he devised his reflecting stereoscope for the purpose of removing the difficulties involved in stereoscopy by direct vision. Figs. 2 and 3 are exact reproductions of his drawings, representing the front view and ground-plan of his original stereoscope; and, in describing them, we can not do better than again to give his own words: "A A' are two plane mirrors, about four inches square, inserted in frames, and so adjusted that their backs form an angle of 90 with each other; these mirrors are fixed by their common edge against an upright, B, or against the middle line of a vertical board, cut away in such manner as to allow the eyes to be placed before the two mirrors. C C' are two sliding boards, to which are attached the upright boards D D', which may thus be removed to different distances from the mirrors. To facilitate this adjustment I employ a right and a left-handed wooden screw, r l; the two ends of this compound-screw pass through the nuts e e', which are fixed to the lower parts of the upright boards D D', so that, by turning the screw-pin p one way, the two boards will approach, and, by turning it the other way, they will recede from each other; one always preserving the same distance as the other from the middle line f. E E' are panels, to which the pictures are fixed in such a manner that their corresponding horizontal lines shall be on the same level; these panels are capable of sliding backward and forward in grooves on the upright boards, D D'. The observer must place his eyes as near as possible to the mirrors, the right eye before the right-hand mirror, and the left eye before the left-hand mirror; and he must move the sliding-panels E E' to or from him, until the two reflected images coincide at the intersection of the optic axes, and form an image of the same apparent magnitude as each of the component pictures.
In using this stereoscope, of which a perspective view is given in Fig. 4, the two conjugate pictures must be on separate cards, but may be much larger than those which are now so extensively used with more modern instruments. The arrangement is obviously such that no side-images can be perceived, since it is impossible for either eye to receive more than one image, and this is reflected from the oblique mirror directly in front. As an instrument it is unwieldy and inconvenient in comparison with those to which we are accustomed; but
Fig. 6.—The Binocular Camera.
with it the great secret of binocular vision was brought into open daylight. Wheatstone had the genius to find out how the door was to be unlocked, and it was left for others to devise the special forms that would be employed in making most acceptable to the world the treasure which he had found. His predecessors had more or less distinct conceptions of an hypothetical treasure, just as something was known about the nature of steam before the low-pressure engine was invented, and about sound-waves before the telephone came into existence. To him distinctly belongs the credit of objectively demonstrating the essential features of binocular vision, with the first instrument actually constructed in accordance with principles which possibly others might have applied, if they had possessed equal clearness of conception and fertility of invention. So slight was the general appreciation of the fact that the two retinal images in binocular vision are dissimilar, that Wheatstone made this discovery independently, and then added the application which others had failed to make, but without the knowledge that any one had preceded him in even forming the conception. The originality of his discovery is not affected by the unemphatic statements afterward found to have been recorded by those who preceded him in thought but not in act.
One of these predecessors was Mr. James Elliot, of Edinburgh, who, "previous to or during the year 1834, had resolved to construct an instrument for uniting two dissimilar pictures." By delay he lost the golden opportunity, which, without envy or knowledge of his existence, was snatched away from him by Wheatstone. Not until 1839 did Elliot construct the instrument which he had contemplated. It was simply a wooden box, open at the extremities, so that a pair of conjugate pictures on glass could be placed at one end, and all light except that which was transmitted through them could be excluded from the eyes placed at the other end. He was not aware of Wheatstone's invention, which indeed did not become generally known for a
Fig. 7.—Brewster's Stereoscope, 1849.
number of years after its completion, because not adapted for general use, and because no other means than free-hand drawing existed for the accurate preparation of the conjugate pictures. Those employed by Wheatstone were outlines of various geometric solids. Elliot's first stereograph was a landscape, represented in Fig. 5, which is a little smaller than that constructed by him. In the background of each picture is the moon, the stereographic interval between them being two and a half inches, which is about the average distance between the pupils of a pair of eyes. Next comes a cross, and in the fore-ground is the withered branch of a tree. In the picture on the right it is seen that the branch is nearly aligned with the cross, which is projected against the sky on one side of the moon; in that on the left one limb of the cross is projected against the moon, while the branch is wholly on the right of both. If the reader will place one edge of a card on the line between the two pictures, while the other edge touches his nose and forehead, he will perceive but a single picture, in which the branch, cross, and moon are successively farther away, the two former standing out in clear relief. By a little attention, moreover, he will see two phantom-cards, one on each side of the combined picture, and between the two his Cyclopean eye is regarding the landscape before him.
At the exhibition of Wheatstone's reflecting stereoscope, and the reading of his paper before the British Association at Newcastle, in August, 1838, one of the most interested auditors present was Sir David Brewster, who remarked on its important bearing upon the
Fig. 8.—Modified Brewster Stereoscope.
theory of single vision to which he himself had given much attention. In his subsequent investigations he devised two important instruments which, with others of less value, were described in papers published in 1849. These were the binocular camera and the lenticular stereo-scope. During the following year they were exhibited in Paris; and here it was that stereoscopy first became the delight of the people after having been confined for a dozen years to the laboratory of the physicist.
The binocular camera needs but little description. Every one is familiar with the instrument, first devised in its simplest form by Baptista Porta, as ordinarily employed by the modern photographer. It consists now of a dark chamber, into which light from the object to be pictured is converged with a combination of carefully corrected achromatic lenses upon a prepared plate whose distance can be readily adjusted. If provided with two such combinations a few inches apart (Fig. 6), so that two pictures of the same object can be simultaneously taken thus from slightly different standpoints, it becomes the instrument on whose coexistence depends the value of the stereoscope. Without it the preparation of the stereograph would be practically impossible in many cases, for a living object, and even many inanimate objects, such as clouds, may move during the interval consumed in changing the position of the single camera and taking the two pictures successively. In the absence of photography dissimilar pictures must be made with the brush or pencil; and, aside from the labor thus imposed, few artists can compete with the sunbeam where perfect accuracy in every detail is required. Without the stereoscope, on the other hand, there would be little or no raison d'être for the binocular camera. Photography can scarcely be said to have had an existence before the publication, in 1839, of the labors of Talbot and Daguerre; and until Archer discovered, in 1851, that collodion could be employed as a vehicle for silver salts, the art was incapable of very wide or successful application for stereoscopic purposes. This epoch in photography, indeed, came after Brewster's double camera had been devised. The latter was itself the timely and natural outcome of the development of this art of sun-drawing, in conjunction with Brewster's invention of a far more convenient form of stereoscope than that employed by his distinguished contemporary. Wheatstone could hardly have entertained any idea of utilizing the evanescent images in silver nitrate obtained prior to 1802 by Wedgwood and Davy, or even those secured in 1814 by the elder Niepce on bituminized plates, which, indeed, were more permanent, but still far from satisfactory. Scarcely a year elapsed after Wheatstone's invention before the first photograph ever obtained from the human face was successfully taken by the leader in photography on our own side of the Atlantic, Dr. John W. Draper; but the art was not yet enough developed, even in such
Fig. 9.—Arrangement of Semi-Lenses.
hands, to suggest the application for stereoscopic purposes which was afterward so happily made by Brewster. To this physicist, therefore, we must credit the invention of the means by which stereoscopy was made to become co-extensive with photography.
The only difficulty in viewing a stereograph, as we have seen, consists in giving the proper direction to the eyes, which, in spite of the efforts of the untrained observer, will generally converge to a single point of fixation. Brewster's mode of preventing this was, like Elliot's, to cause each of the two pictures to be viewed at the bottom of a box, through which light was transmitted. His stereoscope is shown in Fig. 7, which has been taken from an instrument brought to New York in 1850, and much prized by its owner as the first stereoscope ever seen in America. The box is of mahogany, and provided with a lid which can be raised so that an opaque card also may be viewed, if desired, by reflected light admitted from above. The bottom is made of roughened glass so as to diffuse the light that is transmitted, in case a photograph on glass is employed. In either case, the picture can slide easily in and out. To secure the natural convergence of visual lines, a condition which Brewster thought indispensable, a pair of semi-lenses were inclosed in brass tubes at the top of the box. These tubes could be drawn slightly out, like those of an opera-glass, and one was capable of slight lateral motion, being fixed upon a sliding plate of wood as shown in the drawing. They could thus be adapted to different pairs of eyes. They served the double purpose of holding the semi-lenses, with edges toward each other, at the most convenient distance
Fig. 10.—The American Grandfather, 1861.
from the stereograph, and of hiding from each eye the picture intended for the other. Since the rays in transmission are deviated toward the thicker part of the glass, it is possible without discomfort to use pictures on which the stereographic interval exceeds that between the observer's pupils. On ordinary stereographs, however, three inches is the usual limit. Another office performed by the semi-lenses is that of magnifying the pictures as they are binocularly viewed. It was indeed a happy thought that produced such a combination of admirable features.
Much space could be occupied in describing the many forms of stereoscope that have been devised since that of Brewster was first put forth. They have all been applications of the principles already explained in connection with the reflecting and refracting instruments, devised in 1838 and 1849 e That of Helmholtz is probably the best in Europe. In this each tube extends into the box, and is provided with a pair of accurately centred plano-convex lenses, which greatly magnify the pictures. It is indeed simply a pair of telescope eye-pieces, each of which is screwed into a plate to which lateral motion, for the purpose of adjustment, may be given with a screw, lever, and spring. To avoid the necessity of optic divergence, the stereograph must be comparatively small. Such an instrument is necessarily quite costly. The form most widely employed in Europe is that shown in Fig. 8, in which the box is divided by a partition (s), which does not extend so far as to prevent ready motion of the slide. The tubes are discarded and the semi-lenses are permanently fixed, edge to edge (Fig. 9), into the wood at the smaller end. This is objectionable, because no adjustment is possible for either the distance of the card or the width between the eyes.
Twenty years ago the stereoscope just described was the only one extensively used in America. At present it is hard to find, because totally displaced by another instrument, the device of a modest American whose name seems to be but little associated in the popular mind
Fig. 11.—Accommodating Grandchildren, 1882.
with his own invention. This fact would be inexplicable were it not that he has made so many thousands of readers happy by his writings on literary topics that they think of him only as the poet, the professor, the genial "autocrat of the breakfast-table," whose delicate humor and warm human sympathy have so often caused smiles and tears to mingle together, that they forget him as the physiologist, who finds use for other instruments besides his mirth-provoking pen. There are few who think of him as an inventor, when they use the convenient and compact stereoscopes that have been multiplied in tens of thousands, until now no home is too humble, no father too poor, to delight his little ones with phantom scenes of beauty, brought by the sunbeam and the stereoscope from places that their eyes will never behold. The writer will not be deemed blameworthy in transcribing, from a letter that was not intended for the public, a few lines which the author has consented to let him give. Dr. Holmes says: "It appeared to me that the box stereoscopes were cumbrous and awkward affairs. I had one of Smith and Beck's, and one or more of other patterns, but I did not like them; and so one day I cut out a piece of wood in some such shape as this (Fig. 10), the lines representing slots in which the stereograph was to be placed, stuck an awl in for a handle, and there was my stereoscope. . . . I have forgotten to mention the hood, which I made of pasteboard cut to fit. Other open instruments, and many closed ones, have been made, but most of them have been awkward, expensive, and sometimes gimcracky, whereas I think mine may be called simple, strong, cheap, handy."
No better compendium of good qualities can be expressed than is comprised in this brief list of four words. The figure is taken directly from "the original great-grandfather pattern," as the inventor has pleasantly called it, the "real Adam" of hand-stereoscopes, that was born—or developed—in 1861, delighted the human beings who lived in that remote day, and has been sleeping these many years. Compelled now to show itself, like Hip Van Winkle, it is perhaps a little stiff; and in style it is a trifle blunt, in comparison with its polished and accommodating great-grandchildren of the present day (Fig. 11), that fold up and pocket themselves out of sight; but nevertheless its character is that of a straightforward, clear-headed old ancestor, that looks forth honestly from under that somber hood.
To produce the illusion of viewing an actual sunlit scene, Dr. Holmes placed between the stereograph and the semi-lenses an oblique wooden plate, in which were a pair of elliptic openings, so that the effect was that of looking through a circular window. The front was covered with gilt paper from which a golden light was reflected upon the picture. As an appropriate name he selected that of "The Claude Lorraine Stereoscope."
The inventor offered his device gratuitously to manufacturers in New York and Philadelphia, but their refusal was as courteous as was consistent with firm opposition. He did not assume the trouble to secure it by patent, as he "did not care to make money by so obvious and simple a contrivance." A few of these stereoscopes were at last constructed by Mr. Joseph L. Bates, of Boston; and the demand rapidly grew so that now but few of any other make are to be found in the United States. Improvements, indeed, have been added, but not of such kind as to diminish the cost; one of these, introduced by Mr. Bates, was the substitution of a sliding cross-bar for the series of fixed slots. The "Claude Lorraine" effect may be easily obtained with any ordinary stereoscope, by the use of an extra cross-bar, on which the gilt window-plate is hinged; it may thus be adjusted to any position and inclined at will according to the direction from which the light comes.
A simple and moderately satisfactory stereoscope may be improvised by unscrewing the concave eye-pieces from an ordinary opera-glass, and looking through it at the stereograph, which must be held about six inches from the centers of the object-glasses and parallel to the line connecting these. Vision by this method, however, is very uncomfortable if the stereograph be large. The instrument is a crude Helmholtz stereoscope, but it needs adjusting-screws at both ends of each tube to make it entirely satisfactory. The only objection to Dr. Holmes's instrument is the absence of adjustment; but, despite this defect, it is deservedly used everywhere in our country. Quietly and unselfishly he has done far more for the stereoscope in America than has ever been credited him by those who enjoy the fruits of his spontaneous and unpaid ingenuity.