1250 PURKINjti'S FIGURES. [BOOK m. 



knowing the distance k/3 in the diagrammatic eye, the distance 

 /3a can be calculated. But if the distance /3a be thus estimated, 



FlG. 148. DlAGBAM ILLUSTRATING THE FORMATION OP PuRKINJE's FIGURES WHEN THE 



ILLUMINATION is DIRECTED THROUGH THE SCLEROTIC. 



and the distance ba be directly measured, the distances @v, 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 & 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 

 @ where visual impressions begin, on the other. When this is 

 done it is found that the distance ftv thus calculated corresponds 

 fairly well to the distance of the retinal vessels from the layer of 

 rods and cones. Thus Purkinje's figures 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 illumination, even if they do not give absolutely 

 exact results, at least go far to shew that these impulses originate 

 at the outermost 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. 148) 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 movement 

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

 opposite direction to the light when the latter is moved up and 



