VISUAL SENSATIONS. 



761 



down, holding it on a level with the eyes by the side of the head, there will 

 appear in the h'eld of vision of the eye of the same side, projected on the 

 wall, an image of the retinal vessels, quite 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 ; 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 

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

 passing through the sclerotic at b, Fig. 174, in the direction b v, will throw a 

 shadow of the vessel 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 ft to , and the dark line in the field of vision will move from B to A. 

 If the distance B A be measured when the whole image is projected at a 



FIG. 175. 



Diagram illustrating the Formation of 

 Purkinje's Figures when the Illumination 

 is Directed through the Sclerotic. 



Diagram illustrating the Formation of 

 Purkinje's Figures when the Illumination 

 is Directed through the Cornea. 



known distance, k B from the eye, k being the optical centre, 1 then, knowing 

 the distance k ft in the diagrammatic eye,"the distance /3 a can be calculated. 

 But if the distance ft a be thus estimated, and the distance b a be directly 

 measured, the distances /? v, a v,bv,a v 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 



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 princi- 

 pal rays, of the various pencils of rays falling on the lens, pass. The diagrammatic eye 

 of Listing (p. 745) has two optical centres, but these may, without serious error, be further 

 reduced for practical purposes to one lying in the lens near its posterior surface, at about 

 15 mm. distance from the retina. 



