VISION 



999 



on the fovea. (3) When the images of an object do not fall on identical 

 points, one of the points on which they do fall may be occupied with 

 the images of other objects, some of which may be so boldly marked 

 as to enter into conflict with the extra image and to suppress it. 

 (4) Lastly, the physiological ' identical po ; nt ' is not a geometrical 

 point, but an area which increases in size in the more peripheral zones 

 of the retina, and can also be increased by practice; and images which 

 lie wholly or in chief part within two corresponding areas practically 

 coincide. 



Stereoscopic Vision. Although the retinal image is a projection of 

 external objects on a surface, we perceive not only the length and 

 breadth, but also the depth or solidity of the things we look at. When 

 we look directly at the front of a build- 

 ing, the impression as to its form is the 

 same whether one or both eyes be used, 

 although with a single eye its distance 

 cannot be judged so accurately. But 

 when we view the building from such a 

 position that one of the corners is visible, 

 we obtain a more correct impression of 

 its depth with the two eyes. This is 

 partly due to the fact that to fix points 

 at different distances from the eyes the 

 visual lines must be made to converge 

 more or less, and of the amount of this 

 convergence we are conscious througli 

 the contraction of the muscles which 

 regulate it. But there is another element 

 involved. When the two eyes look at a 

 uniformly - coloured plane surface, the 

 retinal image is precisely the same in 

 both. But when the two eyes are 

 directed to a solid object (say a book 

 lying on a table) the picture formed on 

 the left retina differs slightly from that 

 formed on the right, for the left eye sees 

 more of the left side of the book, and 

 the right eye more of the right side. 



That there is a close connection be- 

 tween uniformity of retinal images and 

 impression of a plane surface on the one 

 hand, and difference of retinal images and 

 impression of solidity on the other, is 

 proved by the facts of stereoscopy. It 

 is evident that if an exact picture of the solid object as it is seen by 

 each eye can be thrown on the retina, the impression produced will 

 be the same, whether these images are really formed by the object 

 or not. Now, two such pictures can be produced with a near approach 

 to accuracy by photographing the object from the point of view of 

 each eye. It only remains to cast the image of each picture on the 

 corresponding retina, while the eyes are converged tc the same extent 

 as would be the case if they were viewing the actual object. This is 

 accomplished by means of a stereoscope (Fig. 419). 



It is found that the resultant impression is that of the solid object. 

 It is impossible to reconcile this with the doctrine of strictly identical 

 geometrical points. A pair of identical pictures gives with the stere- 

 oscope not the impression of a solid, but of a plane surface. If the 



Fig. 419. Brewster's Stereoscope. 

 p and TT are prisms, with their re- 

 fracting angles turned towards 

 each other. The prisms refract 

 the rays coming from the points 

 c, 7 of the pictures ab and a/3 so 

 that they appear to come from a 

 single point q. Similarly, the 

 points a and appear to be situ- 

 ated at/, and the points b and /3, 

 at a. 



