CHAP. ii. 



SIGHT. 



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 ; 

 and it is the light emanating from this image which throws shadows 

 of the retinal vessels on to the rest of the retina. 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 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 6, Fig. 70, in the 

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

 (S ; 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. 



B A 



FIG. 70. DIAGRAM ILLUSTRATING THE FORMATION OF PUKKINJE'S FIGURES WHEN THE 

 ILLUMINATION is DIRECTED THROUGH THE SCLEROTIC. 



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

 move from # 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, &B from the eye, k being 

 the optical centre 1 , then, knowing the distance k$ in the diagram- 

 matic eye, the distance fia 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 



1 For the properties of the optical centre, we must refer the reader to the various 

 treatises on optics. The optical centre of a lens is the point through which all the 

 principal rays, of the various pencils of rays falling on the lens, pass. The diagram- 

 matic eye of Listing (p. 492) has two optical centres, but the.e may, without serious 

 error, be further reduced for practical purposes to one lyijg in the lens near its 

 posterior surface, at about 15 mni. distance from the retina. 



F. 33 



