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Stereoscopy 812

eyes, relief, object, distance, equal, stereoscopic and vision

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STEREOSCOPY 812. The Sensation of Relief. We have already seen (§ 30) that the two determining causes of the sensation of relief are the dissimilarity of the two images seen separately by the eyes and the variation in the effort of convergence of the two ocular axes. We have also seen that in proper circumstances, by viewing two perspec tives of the same object, the two viewpoints being a distance apart equal to the separation of the two pupils, we obtain the sensation of seeing the object in relief (stereoscopic vision).1 It is, however, necessary to emphasize at the outset that stereoscopic production of relief is regulated not only by geometric considerations. While these certainly play a primary part, in stereoscopic vision, more even than in binocular vision, there is also a psychical clement in the process, the nature of which is imperfectly understood. When examining the same images in the same stereoscope adjusted each time for the best effect, different observers get very different impressions of the amount of the relief.

A person having eyes symmetrically placed, equal and free from aberration, can see rapidly and without effort the relief of a stereogram (a combination of the two perspectives correspond ing to the two eyes placed side by side) correctly mounted and examined in a properly adjusted stereoscope. However, the adaptabi!ity of the eyes plays an essential part in the appreciation of very slight reliefs, which are often only visible after several minutes' attentive observa tion.

A person whose eyes are unequal or astigmatic sees relief normally if he retains for stereoscopic viewing the correcting glasses necessary for his ordinary vision. But a subject whose defects of vision cannot be corrected (diplopia, strahism, etc.) is obviously as helpless as a one-eyed individual where stereoscopic vision is con cerned.' The distance between the centres of rotation of the two eyes, equal to the distance from centre to centre of the pupils when looking at a distant object, varies greatly from one individual to another, according to race, sex, and age. Its mean value is generally greater for men than women ; in adults it is generally between 2-A in and 3 in. (54 and 76 mm.), the mean adopted

being about 24 in. (63 to 65 mm.).

813. Stereoscopic Photographs. Two photo graphs T and ir of the same object ABCD (Fig. 200) are taken under the same conditions from two points of view 00' (separated by a. distance equal to the mean inter-ocular dis tance) simultaneously, with two identical lenses on two plates similarly situated, or in succession after shifting the camera parallel to the plane of the plates. From the two negatives T and T' are taken two positives T, and T,', and, the eyes occupying the position previously occupied by the lenses, each positive is placed opposite the eye to which it corresponds at a distance OP,, O'P', equal to the principal distance OP, O'P'. It is necessary to give the principal points PP' a separation equal to the separation 00' of the lenses and the eyes ; the image of each point is then situated on the visual ray through the point in question.

Examination of one of these photographs by the corresponding eye (the other being closed) will give, from the point of view of perspective, the same sensation as the object itself. The simultaneous examination of two images will give us the sensation of shape, size, and position which we perceive in examining the object itself, the reconstructed object being identical with the object photographed.

A transparency, being of better gradation and easier to illuminate than a paper print, always gives a more perfect illusion. It may be noted that once the two transparencies are correctly assembled, they can be examined in the four possible positions without modifying the in tensity of relief or the form of the object ; only its direction of presentation being changed.

814. Limits of Perception of Binocular Relief. An observer, viewing a point A situated in front of a plane P (Fig. 201) and closing alternately the two eyes, sees the point A projected alter nately at a„ and Binocular vision shows us the point A detached from the plane P if the distance a (= a,,a,,) is at least equal to the limit of resolution of the eye at the distance in ques tion.

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