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STEREOSCOPE (Gr. (rrepe5s, solid, vxt...

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Originally appearing in Volume V25, Page 897 of the 1911 Encyclopedia Britannica.
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STEREOSCOPE (Gr. (rrepe5s, solid, vxtnrav, to see)). The funda- mental property of stereoscopic vision, or simultaneous vision with both eyes, is the direct perception of the relative distances of near objects. Of course, ideas of the different distances of objects also occur in vision with a single eye, but these are the result of other experiences and considerations. These representations are also not always unequivocal (see fig. 1). For instance they may arise from the former know- ledge of the shape and size of a distant object, from the partial covering of one object by If the head is held still only one portion of space can be observed stereoscopically. The single eye, when moved, surveys, including indirect vision, a field which measures 18o° in a horizontal direction, and 135° in a vertical direction. The two fields overlap and a smaller conical space is formed, with the nose as vertex (B V S in fig. 2), in which both eyes can see simultaneously; and outside this space stereoscopic vision is impossible. The shape and size of this space are very different in men and animals. According to Armin Tschermak the horizontal extent of the space surveyed with both eyes is only 34° in a rabbit as compared with 9o° in man, 15° in a fowl and about 5° in a carp (measured in water). There is a further difference between the eyes of men and animals. The optic axis of the eye is the line joining the centres of the curves, but the direction in which the eye can see most clearly does not always coincide with this, being determined by the spot on the retina which is most susceptible to light, the so-called yellow spot (Fovea, F in fig. 2). In man this spot is still near the axis, although not always exactly on it. It is not perfectly known how it is situated in animals, but in many the axes of the eyes diverge (especially strongly in geese), and the portions of the retina utilized in stereoscopic vision lie far distant from the axis, as in many animals the eyes are only slightly movable. Every time that the eyes are directed on one spot (P in ' The subject of stereoscopy has been extensively developed by the author of this article, who, curiously enough, having lost the sight of one eye through an accident, could no more enjoy the beauties of stereoscopic sight.—ED.itself for a man standing erect and looking straight ahead. All object-points situated outside the horopter fall on points of the retina which are riot identical, but the two images are only seen as real double images in exceptional cases. As a rule the effect is that these points are also seen simply, but at other distances than that of the fixed point P. The differences of the images arise in the moving of the image-points in the direction of the connecting line of the two eyes. For this reason the eyes cannot recognize the space between parallel shining telegraph wires if the connecting line of the two eyes be parallel to the wires, whilst the perception of the depth occurs involuntarily if the connecting line of the eyes is more or less perpendicular to the wires. These differences of images which have been mentioned are therefore necessary and are sufficient for the perception of depth. The explanation that the perception of depth was due to a difference between the two retinal images was first given by Ch. Wheatstone in 1833; but it was contradicted by E. Briicke (1841), Sir David Brewster (1843) and others, who stated that when observing an object the angle of convergence of the axes of the eyes continually changed, and through this and also by the exertion of the muscles and the accommodation of the eye there was a simultaneous touching of the object, which gave rise to the perception of its depth. This latter theory, however, was contradicted by H. W. Dove, who showed that a stereoscopic viewing was also possible with momentary illumination of the object; and still less does it agree with the Chromium fact, to which Wheatstone first called attention, that facsimiles also have a stereoscopic influence, in spite of the fact that the images retain their position on the retina unchanged. Numerous experiments show the same result, and it follows that even a change of the angle of convergence is not always observed as a change of depth. There are two kinds of stereoscopic vision, direct and in-direct, according to whether the point seen indirectly, e.g. H in fig. 3, is compared with the fixed point P, or with another point seen indirectly, e.g. J in fig. 3. In both kinds of stereoscopic vision the exactness of the observation of the depth is greater as the point J approaches H, and the point H approaches P. As a matter of fact a man's eyes are naturally never perfectly still. They move in their sockets, and the point P, where the axes intersect, is continually changing. Direct stereoscopic vision arises from indirect stereoscopic vision and vice versa, and the accuracy of the discernment of the depth increases and decreases. As in this the eye does not revolve round its lens but round the centre of the sphere situated so mm. Jthere is a great difference. In an unchanged focused micro-scope it cannot be distinguished which of the indistinct objects are above and which are below the plane focused for. In stereoscopic vision, however, this can be seen directly. How does this happen? Why does the point H in fig. 3 appear behind and the point V in front of the point P when both eyes are fixed on the point P? As is shown in fig, 3 the image-points on both sides lie further apart for H or nearer together for V than the image-points for P, and for all the points on the horopter (Q, R, S, T &c.), whether the points H and V are situated inside or outside the horopter. In other words, if the point H be formed in the object-space by the moving of the related points Q (or R) towards H, then a movement of the image-point takes place in the right eye (or the left), in both eyes in the direction of the nose, so long as the point H is outside the horopter. On the contrary an external movement of the image-point, i.e. towards the temples, takes place when the points S and T are substituted by the point V situated inside the horopter. This differentiation of the retinal images of the points H and V respectively inside and outside the horopter must suffice, and the question as to how the idea of space is conveyed to the brain is a physiological and psychological subject. If the images of the line PH in both eyes (or of the line PV) arc very different in length, the double images of the point H (or V) are seen without great attention. But the stereoscopic effects are in these cases always the same as before. There is, however, an exception in which the observer sees only two images and in which stereoscopic observation is completely excluded. This exception is important because it occurs in the space in the immediate proximity of P. If for example the second point (H' in fig. 3) is situated behind or in front of the point P, so that it falls between the two optic axes, or on one of them, then only double images can be seen, either of P or of H', according to whether the optic axis cuts at P or H', or double images of both points if the optic axes intersect at any other point of the line PH', but the representation of the difference of depth of the two points P and H' is never obtained. This fact can be easily realized if a stick, e.g. a lead pencil, be held before the eyes of an observer with good stereoscopic sight so that its lengthwise axis falls exactly on a point between the eyes or in the middle of one of the two eyes. The double images can he seen still more clearly if two small balls on thin threads are suspended behind one another so that their connecting line retains the position mentioned above. In this experiment it can be seen directly how inconvenient these double images are to the observer. He involuntarily tries to evade them by moving the head. The reason for this is that, when P (or H') is fixed, the images of H' (or P) are always separated from one another by the centre of the yellow spot. The distances of the two images from the yellow spot have consequently opposite signs, whilst for all other objects (e.g. H) which lie outside the two axes the distances have the same signs. The difference of the sign is, however, not alone decisive, for if the connecting line PH' is moved a little higher or lower out of the plane FPF the signs remain different, but the stereoscopic effect is immediately regained. "Therefore in all cases in which the connecting line PH' is seen with one eye as a point and with the other as a line, or with both eyes as a line, but from two diametrically opposite sides, there is no stereoscopic effect, but double images are seen; and that for stereoscopic observation it is essential to see the connecting line PH' with both eyes simultaneously from one and the same side, from above or below, from the left or the right. This condition is provided for in the stereotelemeter by the arrangement of a zigzag measuring scale, so that the connecting-line of the marks slightly ascends. Care must be taken when using this instrument (as also when using any stereoscopic measuring instrument) that the index hangs close to or above the object to be measured, so that the latter is only touched and in no way covered by the mark. The power of perception of depth in man is most accurate. This has been ascertained by the approximately equal keenness of vision of all normal-sighted people and by the interpupillary distance. The angle which serves as a measure for the keenness of vision is that under which appear two neighbouring points of the object-space which are still seen by the single eye as a double point; according to the older experiments of Helmholtz, this angle is about 1'. When measured on the retina the keenness of vision is determined by the diameter of the nerve filaments situated in straight rows close to one another in the fovea (fig. 4). The diameter of these filaments amounts to roughly o•oos mm., or in angular measure r'. More recent experiments for keenness of vision and power of perception of depth have given considerably higher values (Wiilfing, Pulfrich, Heine and others); thus Pulfrich in 1899, when first introducing behind it, the entrance-pupil of the eye moves slightly to and fro and up and down, and many experiments have been made to produce a perception of depth for a single eye from the relative movements of the images consequent on this motion. As these movements of the images only occur in indirect vision, it can be understood they are not seen by most people. 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. If the attention and eyes are directed upon a certain object, as, for instance, in manual labour and in measuring the image-space with the so-called " travelling mark " on the stereo-comparator, then direct stereoscopic vision only is concerned. Stereoscopic vision is in many ways similar to the monocular observation of a preparation under the microscope, and yet stereoscopic instruments for measuring distance, proved that as a rule persons with normal eyes have a power of perception of depth of so" and still less in unrestricted vision. This is explained as follows (Hering, Heine): It is unimportant for perception where the filament mentioned above is illuminated. In order to see two objects lying close to one another it is not essential that the two image-points should be separated from one another by the distance of the two nerve filaments of the eyes. This happens whenever the line separating two objects passes through the two points (see fig. 4). It is natural that the perception of depth has no fixed limits, for the position of the images shown in fig. 4 changes with the movement of the eyeball, and the closer the two points are to one another, the more rarely it occurs. If the angle of convergence of the optic axes =A, the (average) distance between the eyes B=65 mm., 6=2' relatively = 1:7000 (the perception of depth easily attained by normal sight) and E=the normal distance of the point P from B in fig. 2, then from E=B/A, the change of depth dE gives: dE = B . 6/A2 = E .6/A = E2.6/B. If the angle A has the value 6 then all perception of depth ceases. At this distance objects are only still distinguishable from those lying behind them, which together form a surface but cannot always be seen as a surface because our representations of the depths of distant objects are not conclusively controlled by stereoscopic sight. This distance is called the radius of the stereoscopic field, and is calculated by the formula R=B/6, whence R=45o metres. From the above formulae it can be directly seen that the variation dE increases with E' and the proportional variation dE/E increases with E. The numerical values can be easily calculated when either A or P. is given thus: dE/E = 6/A or dE/E = E/R. The limits of stereoscopic vision defined above can be extended and under the name of " stereoscope " every binocular instrument is included which serves this end. Those instruments should first be mentioned which have restored the more or less lost power of stereoscopic vision. It is necessary for those with normal sight to wear spectacles when the eyes cease to accommodate themselves to objects near at hand. Spectacles which only cover the lower half of the eye and leave the upper A, A; half free to look out into space are the best. For those who have been operated on for cataract, and for excessively short-sighted persons, the " telescope-spectacles " devised by M. v. Rohr (of Zeiss, Jena) are a great assistance. There are two
End of Article: STEREOSCOPE (Gr. (rrepe5s, solid, vxtnrav, to see))
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