930 



A MANUAL OF PHYSIOLOGY 



bright field when the direction of the illuminating beam is altered. 

 Generally opacities in the vitreous humour are movable, in the lens 

 not. 



Purkinje's Figures. As was first pointed out by Purkinje, 

 the shadows of the bloodvessels in the retina itself, and even 

 of the corpuscles circulating in them, although neglected in 

 ordinary vision, may be recognised under suitable conditions, 

 a conclusive proof that the sensitive layer must lie behind the 

 vessels (p. 932). 



If a beam of sunlight is concentrated on the sclerotic as far as 

 possible from the margin of the cornea, and the eye directed to a 



dark ground, the net- 

 work of retinal blood- 

 vessels will stand out 

 on it. Another method 

 is L to look at a dark 

 ground while a lighted 

 candle, held at one 

 side of the eye at a 

 distance from the 

 visual line, is moved 

 slightly to and fro. 

 In j the first method, 

 a point of the sclerotic 

 behind the lens is illu- 

 minated, and rays 

 passing from it across 

 the interior of the 



eyeball in every direc- 

 FIG. 408. METHOD OF RENDERING THE RETINAL 



BLOODVESSELS VISIBLE BY CONCENTRATING A 

 BEAM OF LIGHT ON THE SCLEROTIC. 



From the brightly-illuminated point of the scle- 

 rotic, 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 



tion cast shadows of 

 the vessels of the re- 

 tina on its sensitive 

 layer. In the second 

 method, the image of 

 the flame formed on 



nodal point. When the light is shifted so as to be the re ^ na b 7 r f / s f a11 ,- 

 focussed at b, the shadow cast at V is referred to in g obliquely through 

 b" i.e., it appears to move in the same direction as tne pupil becomes in 

 the illuminated point of the sclerotic. the general darkness 



itself a source of light, 



by interrupting the rays from which the 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 position 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 sensitive 

 layer is about o'2 to 0-3 mm. behind the stratum which contains the 

 bloodvessels. This corresponds sufficiently well with the position 

 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 

 of the blood-corpuscles in the retinal vessels may be rendered visible 

 by looking at a bright and uniformly illuminated ground, like the 



