918 PUEKINJ^'S FIGUKES. [BOOK in. 



ma tic eye, then, knowing the distance kj3 in the diagrammatic 

 eye, the distance fBa can be calculated. But if the distance (3a 

 be thus estimated, and the distance ba be directly measured, the 

 distances /3v, av, bv, av can be calculated ; and if the appearance 

 in the field of vision is really caused by the shadow of v falling 

 on /3, these distances ought to correspond to the distances of the 

 retinal vessels v from the sclerotic b on the one hand and from 

 that part of the retina /3 where visual impressions begin, on the 



FIG. 152. DIAGRAM ILLUSTRATING THE FORMATION OF PURKINJE'S FIGURES 

 WHEN -THE ILLUMINATION is DIRECTED THROUGH THE SCLEROTIC. 



other. When this is done it is found that the distance fiv thus 

 calculated corresponds fairly well to the distance of the retinal 

 vessels from the layer of rods and cones. Thus Purkinj<Ts fig- 

 ures prove in the first place that the sensory impulses which 

 form the commencement of visual sensations originate in some 

 part of the retina behind the retinal vessels, i.e. somewhere 

 between them and the choroid coat; and calculations based on 

 the movements of the shadows following movements of the illu- 

 mination, even if they do not give absolutely exact results, at 

 least go far to shew that these impulses originate at the outer- 

 most part of the retina, viz. the layer of rods and cones. 



In the second method of experimenting, where the light passes 

 through the sclerotic, the image always moves in the same direc- 

 tion as the light, as it obviously must do, when the spot of light 

 on the sclerotic is moved from a to b (Fig. 152) the shadow on 

 the retina moves from a to /3, and the (inverted) image moves 

 from A to B. In the first method, where the light enters through 

 the cornea, the image moves in the same direction as the light 

 when the light is moved from side to side, provided the move- 

 ment does not extend beyond the middle of the cornea, but in 



