430 ELEMENTARY PHYSIOLOGY 



optic nerve are themselves insensible to it. The evidence 

 on which this statement rests is : — 



(i) The blind spot is full of nerve fibres, but has no 

 cones or rods. 



(ii) The yellow spot, where the most acute vision is 

 situated, is full of close-set cones, but has no nerve 

 fibres. 



(iii) If one goes into a dark room with a single small 

 bright candle, and, looking towards a dark wall, moves 

 the light up and down, close to the outer side of one eye, 

 so as to allow the light to fall very oblicjuely into the eye, 

 one of what are called Purkinje's figures is seen. 

 This is a vision of a series of diverging, branched, dark, 

 sometimes reddish, lines on an illuminated field, and in 

 the interspace of two of these lines is a sort of cup-shaped 

 disc. The branched lines are the images of shadows 

 thrown by the retinal blood-vessels, and the disc is that 

 of the shadow thrown by the edge of the yellow spot. 

 As the candle is moved up and down, the lines shift their 

 position, as shadows do when the light which throws them 

 changes its place. 



Now, as the light falls on the inner face of the retina, 

 and the images of the vessels to which it gives rise shift 

 their position as it moves, whatever constitutes the end- 

 organ, through which light stimulates the fibres of the 

 optic nerve, must needs lie on the other, or outer, side of 

 the vessels. But the fibres of the optic nerve lie among 

 the vessels, and the only retinal structures which lie out- 

 side them are the nuclear layers and the rods and cones. 



(iv) Just as, in the skin, there is a limit of distance 

 within which two points give only one impression, so there 

 is a minimum distance by which two points of light falling 

 on the retina must be separated in order to appear as 

 two. And this distance corresponds pretty well with the 

 diameter of a cone. 



The image of the retinal blood-vessels may be also very 

 readily seen by looking at a bright surface, such as the 



