VISION 



1037 



on ojle side, and on account of the crossing of the rays this illuminated 

 portion will be on the same side of the principal axis as the image ol 

 the source of light (Fig. 437). When the image of the source of light is 

 moved to the right the light area of the observed pupil will also move 

 to the right i.e., with rotation of a concave mirror in the same direction 

 as the image of the source of light, and with rotation of a plane mirror 

 in the opposite direction (Snellen). 



A method of photographing the retina in the living eye has also been 

 employed as a means of investigating the fundus. 



Single Vision with Both Eyes Diplopia. Scheiner's experiment shows 

 that it is possible to have double vision, or diplopia, with a single eye 

 when two separate images of the same object fall upon different parts 

 of the retina. In vision with both eyes, or binocular vision, an image 

 of every object looked at is, of course, formed on each retina, and we 

 have to inquire how it is that as a rule these images are blended in 

 consciousness so as to produce the perception of a single object; and 



A 



Fig. 437. Path of Rays in Skiascopy (Myopic Eye) (Snellen). PR, far -point ol 

 observed eye. The other references are as in Fig. 436. 



how it is that under certain conditions this blending does not take 

 place, and diplopia results. Two chief theories have been invoked in 

 the attempt to answer these questions: (i) the theory of identical 

 points, (2) the theory of projection. 



In regard to the second theory, we shall merely say that it assumes 

 that in some way or other the retina, or, rather, the retino-cerebral 

 apparatus, has the power of appreciating not only the shape and size 

 of an image, but also the direction of the rays of light which form it, 

 and that the position of the object is arrived "at by a process of mental 

 projection of the image into space along these directive lines. Where 

 the directive lines of the two eyes cut each other, the two images coin- 

 cide, and the object is seen single in the position of the point of inter- 

 section. The first theory we shall examine in some detail. 



The Theory of Identical Points. This theory assumes that every 

 point of one retina ' corresponds ' to a definite point of the other retina, 

 and that in virtue of this correspondence, either by an inborn necessity 

 or from experience, the mind refers simultaneous impressions upon two 

 corresponding or identical points to a single point in external space. 

 If we imagine the two retinae in the position which the eyes occupy 

 when fixing an infinitely distant object that is, with the visual axes 

 parallel to be superposed, with fovea over fovea, every point of the 

 one retina will be covered by the corresponding point of the other 

 retina, so that identical points could be pricked through with a needle. 



