Stereopticon

“ The instrument first devised by Professor Wheatstone, was what is termed a reflecting stereoscope, and was expensive to make and cumbrous to use. It was modified by Sir David Brewster, by the substitution of prisms for reflectors, and was thus made cheaper and more portable. The refracting form of stereoscope is so familiar to most people, that it really needs no description. It will only be necessary to mention that the prisms used in the eye-pieces are made by cutting a double convex lens in two, and reversing the halves. ”

“ The impossibility of the stereograph's perjuring itself is a curious illustration of the law of evidence. "At the mouth of two witnesses, or of three, shall he that is worthy of death be put to death; but at the mouth of one he shall not be put to death." No woman may be declared youthful on the strength of a single photograph; but if the stereoscopic twins say she is young, let her be so acknowledged in the high court of chancery of the God of Love.
Some two or three years since, we called the attention of the readers of this magazine to the subject of the stereoscope and the stereograph. ”

“ For the sake of clearness, I denominate the angle formed from the focal point of lens, and the glass at back of camera, the angle of delineation; the said glass the plane of delineation and the angle formed by the stereograph to the eye, the stereoscopic angle. It must be borne in mind that the stereoscopic angle is that subtended by one stereograph and the eye. I find by experiments that the angle of delineation is very often larger than the stereoscopic angle, so that the apparent enlargement spoken of by Mr. Shadbolt does not often exist ”

“ The reason for making the separation between the cameras greater than that between the two eyes, is exceedingly simple. The stereograph is to be looked at much nearer than the object itself, and consequently is to be seen under a much larger angle than it is viewed by the two eyes in nature. Hence the two pictures should be taken at the angle under which they are to be observed in the stereoscope. Suppose the object to be 50 feet distant, then of course it is seen by the two eyes under an angle of 21⁄2 inches in 50 feet, or 1 in 240. ”

“ Wilkinson: "I obtain all the information I can from every source; then try, and judge for myself." Hence the present letter.
I regret to be obliged to differ from Mr. Shadbolt, but there is a point in his communication which appears to me to arise from a misconception of the stereoscopic problem. He says (p. 557.) , "for distant views there is in nature scarcely any stereoscopic effect." Now, surely visual distance is merely visual stereosity; for, to see an object solid is merely to see its parts in relief, some of them appearing to project or recede from the others. ”

“ We have already seen, in the history of the stereoscope, that in the binocular vision of objects, each eye sees a different picture of the same object. In order to prove this, we require only to look attentively at our own hand held up before us, and observe how some parts of it disappear upon closing each eye. This experiment proves, at the same time, in opposition to the opinion of Baptista Porta, Tacquet, and others, that we always see two pictures of the same object combined in one. ”

“ Such are the stereoscope and the photograph, by the aid of which form is henceforth to make itself seen through the world of intelligence, as thought has long made itself heard by means of the art of printing. The morphotype, or form-print, must hereafter take its place by the side of the logotype, or word-print. The stereograph, as we have called the double picture designed for the stereoscope, is to be the card of introduction to make all mankind acquaintances. ”

“ ALL of the forms under which the stereoscope has come into general use have been devised with a view to creating to the utmost the illusion of natural binocular perspective by reproducing as nearly as possible the conditions of natural vision. That this end is not successfully attained is painfully felt by those who linger too long over an attractive collection of stereographs. To secure comfortable vision the muscles of the eyes must suffer no unusual strain. ”

“ The faculty of seeing stereoscopic slides single without an instrument is of the utmost utility to the student of physiological optics, and persons with strong eyes can easily acquire it. The only difficulty lies in dissociating the degree of accommodation from the degree of convergence which it usually accompanies. If the right picture is focussed by the right eye, the left by the left eye, the optic axes must either be parallel or converge upon an imaginary point some distance behind the plane of the pictures, according to the size and distance apart of the pictures. ”

“ With the primary position of the eyes as long as the two different retinal views are sufficiently similar to allow a synthesis in a three-dimensional impression of our object, we perceive every point of the object not as double image but as one point of a given distance. The distance feeling of the normal stereoscopic vision demands thus itself more than the reference to the different retinal images, and the only factor which can explain the phenomena is the response of the eye-muscles which react on the double images by increase or decrease of convergence. ”

“ When the stereoscope is perfectly adapted to the eyes of the observer, and the general union of the pictures effected, the remote parts of the picture, that is, the objects seen in the distance, may be under refracted, while those in the foreground are over refracted, so that while eyes which have the power of convergence beyond the picture, unite the more distant objects which are under refracted, they experience much difficulty in uniting those in the foreground which are over refracted. ”

“ This influence over our judgment is beautifully shewn in viewing, with one eye, photographs either of persons, or landscapes, or solid objects. After a little practice, the illusion is very perfect, and is aided by the correct geometrical perspective and chiaroscuro of the Daguerreotype or Talbotype. To this effect we may give the name of Monocular Relief, which, as we shall see, is necessarily inferior to Binocular Relief, when produced by the stereoscope. ”

“ If the eyes are not equal in focal length, that is, in the distance at which they see objects most distinctly; or if, from some defect in structure, they are not equally good, they will still see the stereoscopic relief, though the picture will be less vivid and distinct than if the eyes were in every respect equal and good. There are many persons, however, whose eyes are equal and perfect, but who are not able to unite the pictures in the stereoscope. ”

“ A friend complained to me that the stereoscope she had bought, on my recommendation, was worthless. I had taken it on faith, supposing that all "Holmes" instruments must be good; but a short examination showed that the glasses were rather flat, and were placed so near together that the rays came to the eyes through the central part of the lens, and of course without refraction, and hence the eyes were not deceived. ”

“ If the pictures are refracted too little, or if they are refracted too much, so as not to be united, their tendency to unite is so great, that they are soon brought together by the increased or diminished convergency of the optic axes, and the stereoscopic effect is produced. Whenever two pictures are seen, no relief is visible; when only one picture is distinctly seen, the relief must be complete.
In the preceding diagram we have not shewn the refraction at the second surface of the lenses, nor the parallelism of the rays when they enter the eye,—facts well known in elementary optics. ”

“ Every picture, whether taken photographically or by the eye, is necessarily a picture seen by one eye, or from one point of sight; and, therefore, a skilful artist, who fully understands the principle of the stereoscope, might make a copy of any picture as seen by the other eye, so approximately correct as to appear in relief when united with the original in the stereoscope; but the task would be a very difficult one, and if well executed, so as to give a relievo without distortion, the fortune of the painter would be made. ”

“ If the attention be carefully directed successively to the foreground and background when binocularly viewing a properly constructed outline stereograph, it will be found that perfectly distinct vision of the whole picture at any given moment is not usually possible. The distance between corresponding background points exceeds that between similar foreground points. This excess we shall call the stereoscopic displacement. ”

“ If every one had the same inter- pupillary distance there would be nothing more perfect than this stereoscope.
If in fig. 6 the two pictures L and R are interchanged in both pictures (a or b in fig. 11) , then the image-points for H are closer together than those for V; thus in stereoscopic vision H appears in front of P, and V behind it. No change is made to the relief by turning the picture upside down (c and d in fig. 11) . In fig. lid, the pictures are in the same positions as when the photographs are taken (Fi, F 2 in fig. 6) . Obviously transparent pictures can be easily reversed ”

“ When the binocular pictures are taken at too great an angle, so as to produce a startling amount of relief, the distance between similar points in each picture, both in the distance and in the foreground, is much greater than it ought to be, and hence the difficulty of uniting the pictures is greatly increased, so that persons who would have experienced no difficulty in uniting them, had they been taken at the proper angle, will fail altogether in bringing them into stereoscopic relief. ”

“ But with ordinary stereoscopic pictures this object needs to be very far off, so that this contrivance is
not a very convenient one; nor is it desirable to make a hole in the centre of every picture we wish to see stereoscopically with our naked eyes. Another method is to make one familiar with the muscular sensibilities of the eyes according as their axes converge or diverge, and to make him acquire such control of his eyes that, without looking at any distant object, he will be able to place the axes of the eyes parallel. ”

“ We, who have exhausted our terms of admiration in describing the stereoscopic picture, will not quarrel with the common taste which prefers the card-portrait. The last is the cheapest, the most portable, requires no machine to look at it with, can be seen by several persons at the same time,—in short, has all the popular elements. Many care little for the wonders of the world brought before their eyes by the stereoscope; all love to see the faces of their friends. ”

“ By closing the left eye we shall see merely the portrait b, having, like the other, only monocular relief, but a relief greater than the best-painted pictures can possibly have, when seen even with one eye. [Pg 68] When we open both eyes, the two portraits instantly start into all the roundness and solidity of life.
Many persons experience a difficulty in seeing the portraits single when they first look into a stereoscope, in consequence of their eyes having less power than common over their optic axes, or from their being more or less distant than two and a half inches, the average distance. ”

“ With this figure the reader may attempt the experiment of seeing the stereoscopic effect without the stereoscope. When he has succeeded in doing this, or when he fuses the images together by placing a simple stereoscope over the page, he will find the result very singular; for he will receive the impression of a solid crystal of some dark polished substance—black lead, for instance—placed on a surface of the same material. The edges of the solid will appear to have a certain lustre, such as one sees on the edges of a real crystal. ”

“ The impression of the relief appears when each eye sees that one of the two images which presents the perspective that it would perceive if it saw the real object. If we take two transparent stereoscopic images and place each of them in a projection lantern, in such a way that they can be superposed upon the screen, we shall obtain thereby a single image. It will always be a little light and soft, as the superposition cannot be effected accurately, the perspective not being the same for each of them. ”

“ What is this problem? for until that be known, we cannot hope for a solution. I would ask, is it this?—Stereoscopic pictures should create in the mind precisely such a conception as the two eyes would if viewing the object represented by the stereograph. If this be the problem (and I cannot conceive otherwise) , its solution is simple enough, as it consists in placing the cameras invariably 21⁄2 inches apart, on a line parallel to the building, or a plane passing through such a figure as a statue, &c. ”

“ If, while viewing the combined image, the semi-lenses be screwed closer together, the eyes will continue to adapt themselves, while fusion of images is retained, and any degree of divergence is thus induced that the observer may be disposed to endure. If the stereograph has been properly selected to illustrate the effects of muscle-reading, the image will appear to increase in depth as the visual lines diverge. ”

“ This, however, cannot be regarded as an actual perception of depth, because these viewings necessitate a consideration for each individual interpretation, which is quite foreign to stereoscopic vision.
Indirect stereoscopic vision is of great importance. It makes it possible to recognize any sudden danger or obstacle outside the direction in which one is looking. Even with the stereo- telemeter (see below) the position of the range through which, for example, a bird flies, could not always be accurately given, if one were solely dependent upon direct stereoscopic vision. ”