VISION. 



1-19 



lie in a certain circle, designated the horopter; 

 a circle which passes at once through the 

 point of coincidence, /, of the visual axes, la, 

 I b, and the points of decussation, c c', of 

 these axes with the lines of direction. 



Fig. 837. 



Let c c' be the centres of the eyes A B, / 

 the point regarded, Ice' the horopter, and 

 b the points of the retina on which the axes 

 of the eyes terminate ; and let m be a second 

 point in the horopter. The point / appears 

 upon the axial points a and b, the point m at 

 o and p ; a line betwixt / m will form the 

 chord of the arc of the horopter lying between 

 / and m ; and as all triangles drawn upon the 

 arc of a circle have equal peripheral angles, 

 so is the angle / c m equal to the angle I c' m. 

 Both are farther equal to the opposite angles 

 oca, pc'b. Moreover o c is equal to p c, 

 and a c to b c', as radii proceeding from the 

 centres c and c' of the retinal circles, which 

 in each eye have an equal circumference. 

 Consequently o is just as far from the axial 

 point a, as p is from the axial point b ; and so 

 identical or corresponding points of the retina 

 are affected by the rays proceeding from both 

 / and m. It is, however, to be borne in mind, 

 that though impressions made on non-cor- 

 responding points of the middle portions of 

 the two retinae are perceived as two, impres- 

 sions made on non-corresponding points of 

 the circumferential parts are not so perceived, 

 the distance between such points being within 

 certain limits. 



When an object is viewed at so great a 

 distance that the optic axes of both eyes are 

 sensibly parallel when directed towards it, the 

 perspective projections of it seen by each eye 

 separately are similar, and the appearance to 

 the two eyes is precisely the same as when 

 the object is seen by one eye only. But this 

 similarity no longer exists when the object 

 is placed so near the eyes that to view it the 

 optic axes must converge : under these con- 

 ditions a different perspective projection of it 

 is seen by each e\e ; and these perspectives 



are more dissimilar as the convergence of the 

 optic axes become sgreater. Fig. 888. repre- 

 sents the two perspective projections of a 

 cube. B is that seen by the right eye, and A 

 that presented to the left eye, the figure being 

 supposed to be placed about seven inches im- 

 mediately before the spectator and viewed 

 with eacli eye alternately, the other being 

 closed, and the head kept perfectly steady. 



Fig. 888. 



B 



\ 



Mr. Wheatstone has shown that the single 

 sensation excited by these two images is that 

 of a third image different from them both, but 

 excitable only by both of them at once, and 

 attended with the notion of solidity or projec- 

 tion in relief. This he has illustrated by a 

 most ingenious instrument called the stereo- 

 scope. Accurate representations are drawn 

 of the appearance presented by an object of 

 three dimensions, when viewed by each eye 

 at a short distance. These drawings are then 

 placed symmetrically in the right and left 

 compartments of a small box, so as to be 

 reflected by sloping mirrors to the eyes 

 of the observer, who must place them as near 

 as possible to these mirrors ; then, by moving 

 sliding panels to or from him, he will find 

 a position, and one only, in which the bin- 

 ocular image will be seen single, of its proper 

 magnitude, and without fatigue to the eye, 

 because in this position only the ordinary re- 

 lation between the magnitude of the pictures 

 on the retina, the inclination of the optic 

 axes, and the adaptation of the eye to distinct 

 vision at different distances, are preserved. It 

 being thus shown that there is an essential 

 difference in the appearance of objects when 

 seen with both eyes, and when only one eye is 

 employed; and that the most vivid belief of 

 the solidity of an object of three dimensions 

 arises from two different perspective projec- 

 tions being simultaneously presented to the 

 mind, the question arises, how is it that per- 

 sons who see with only one eye form correct 

 notions of solid objects, and never mistake 

 them for pictures ? and how happens it that 

 a person having the perfect use of both eyes 

 perceives no difference in objects around him 

 when he shuts one of them ? To explain 

 these apparent difficulties, says Mr. Wheat- 

 stone, it must be kept in mind that, although 

 the simultaneous vision of two dissimilar pic- 

 tures suggests the relief of objects in the most 

 vivid manner, yet there are other signs which 

 suggest the same ideas to the mind, and are 

 less liable to lead the judgment astray in pro- 

 portion to the extent of our previous expe- 

 rience. The vividness of relief arising from 

 the projection of two dissimilar pictures, one 



