THE SENSES 



811 



dark ground, the network of retinal bloodvessels will stand out on it. 



Another method is to look at a dark ground while a ligLted candle, 



held at one side of 



the eye at a distance 



from the visual line, is 



moved slightly to and 



fro. In the first 



method, a point of the 



sclerotic behind the 



lens is illuminated, 



and rays passing from 



it across the interior 



of the eyeball in every 



direction cast shadows 



of the vessels of the 



retina on its sensitive 



layer. In the second 



method, the image of 



triP flam^ fnrmpH on 



the flame formed on 

 the retina by rays fall- 



ing obliquely through 

 the pupil becomes in 



. 



FlG> S"- METHOD OF RENDERING THE RETINAL 



BLOODVESSELS VISIBLE BY CONCENTRATING A 

 BEAM OF LIGHT ON THE SCLEROTIC. 



From the brightly illuminated point of the sclerotic, 

 a, rays issue, and a shadow of a vessel, v, is cast at a'. 

 It is referred to an external point a" in the direction of 

 -.. , the straight line joining a with the nodal point. When 



itself a source Of light, the light is shifted so as to be focussed at b, the shadow 



by interrupting the cast at & ' s referred to b", i.e., it appears to move in the 



rays from which the Same direction M th illuminated point of the sclerotic. 



retinal vessels form shadows. 



The distance of the sensitive 



from the vascular layer may 



be approximately calculated 



by measuring the amount by 



which the shadows change 



their position, when the posi- 

 tion of the illuminated point 



of the sclerotic is altered. 



The nearer a vessel lies to 



the sensitive layer, the smaller 



must be the angle through 



which the apparent position 



of its shadow moves for a 



given movement of the spot 



of light. In this way it has 



been calculated that the sensi- 

 tive layer is about 0'2 to Light from a candle at a illuminates a', and 



0"* mm. behind the Stratum rays proceeding from a' cast a shadow of the 

 - - bloodvessel v at a", which is referred to a"'. 

 When a is moved to b, the shadow on the 

 retina moves to b" ', and the shadow in the 

 visual rield of the illuminated eye to b"'. 



BY OBLIQUE ILLUMINATION THROUGH 

 THE CORNEA. 



which contains the blood- 

 vessels. This corresponds 

 sufficiently well with the posi- 



tion of the layer of rods and cones, which all other evidence shows to 

 be the portion of the retina actually stimulated by light. The shadows 



