CHAP, in.] SIGHT. 917 



may be calculated from the diagrammatic eye ( 527). The 

 position thus determined coincides exactly with the entrance of 

 the optic nerve, and the dimensions (about 1-5 mm. diameter) also 

 correspond ; the exact size and shape of the blind spot differs 

 however in different individuals. While drawing the outline 

 as above directed the indications of the large branches of the 

 retinal vessels as they diverge from the entrance of the nerve 

 can frequently be recognized. The existence of the blind spot 

 is also shewn by the fact that an image of light, sufficiently 

 small, thrown upon the optic nerve by means of the ophthal- 

 moscope, gives rise to no sensations. 



The existence of the blind spot proves that the optic fibres 

 themselves are insensible to light, that light can stimulate them 

 only through the agency of the retinal structures- in which they 

 end. 



574. Purkinj^s Figure's. If one enters a dark room with 

 a candle and while looking at a plain (not parti-coloured) wall, 

 moves the candle up and down,, holding it on a level with the 

 eyes by the side of the head, there will appear *in the field of 

 vision of the eye of the same side, projected on the wall, an 

 image of the retinal vessels, similar to that seen on looking into 

 an eye with the ophthalmoscope. The field of vision is illumi- 

 nated with a glare, and on this the branched retinal vessels 

 appear as shadows. In this mode of experimenting the light 

 enters the eye through the cornea, and an image of the candle 

 is formed on the nasal side of the retina ; it is the light emanat- 

 ing from this image which throws shadows of the retinal vessels 

 on to the rest of the retina. In Fig. 153 the light a forms an 

 image on the retina at b ; the light reflected from this spot casts 

 a shadow of the retinal vessel v on to another part of the retina 

 at <?, and the image of this shadow appears in the field of vision 

 at d. A far better method is for a second person to concentrate 

 the rays of light, with a lens of low power, on to the outside 

 of the sclerotic where this is thin just behind the cornea; the 

 light in this case emanates from .the illuminated spot on the 

 sclerotic and passing straight through the vitreous humour throws 

 a direct shadow of the vessels on to the retina. Thus the rays 

 passing through the sclerotic at 5, Fig. 152, in the direction 5i/, 

 will throw a shadow of the vessel v on to the retina at /3 ; this 

 will appear as a dark line at B in the glare of the field of vision. 

 This proves that the structures in which visual impulses origi- 

 nate must lie behind the retinal vessels, otherwise the shadows 

 of these could not be perceived. 



If the light be moved from b to a, the shadow on the retina 

 will move from /3 to a, and the dark line in the field of vision 

 will move from B to A. If the distance BA be measured when 

 the whole image is projected at a known distance, kE from the 

 eye, k being the nodal point ( 527) of the reduced diagram- 



