CHAP. IIL] SIGHT. 1249 



the size as well as the position of the area of the retina cor- 

 responding to the blind spot may be calculated from the 

 diagrammatic eye ( 705). The position thus determined coin- 

 cides 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 recog- 

 nized. 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 ophthalmoscope, 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. 



771. Purkinje's Figures. 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 illuminated 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 emanating from this image which throws 

 shadows of the retinal vessels on to the rest of the retina. In 

 Fig. 149 the light a forms an image on the retina at 6; the light re- 

 flected from this spot casts a shadow of the retinal vessel v on to 

 another part of the retina at c, 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 ( 712) 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 b, Fig. 148, 

 in the direction bv, will throw a shadow of the vessel v on to the 

 retina at @ ; 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 originate 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 (B to <z, 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, &B from the eye, k being 

 the nodal point ( 705) of the reduced diagrammatic eye, then, 



