804 A MANUAL OF PHYSIOLOGY 



as in the direct method to cause dilatation of the pupil by 

 atropia, which also relaxes the accommodation. (Practical 

 Exercises, p. 861.) 



A method of photographing the retina in the living eye 

 which has recently been employed with success by Dogiel 

 bids fair to become an important supplementary means of 

 investigating the fundus. 



Single Vision with Both Eyes Diplopia. Schemer's experi- 

 ment 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. 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 



