THE SENSES 



923 



light only on one 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 of the source of light (Fig. 402) . 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 



FIG. 402. PATH OF RAYS IN SKIASCOPY (MYOPIC EYE) (SNELLEN). 

 PR, far-point of observed eye. The other references are as in Fig. 401. 



A method of photographing the retina in the living eye which has 

 recently been employed with success bids fair to become an important 

 supplementary means of investigating the fundus (Dogiel). 



Single Vision with Both Eyes Diplopia. Schemer'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 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 coincide, and the object is seen single in the position of the 

 point of intersection. 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 impres- 

 sions 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 objectthat is, 

 with the visual axes parallel to be superposed, with fovea over 



