VISION 



1043 



tive layer. In the second method, the image of the flame formed on 

 the retina by rays falling obliquely through the pupil becomes in 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 move- 

 ment of the spot of light. In this way it has been calculated that the 

 sensitive layer is about 0-2 to 0-3 mm. behind the Stratum which con- 

 tains the bloodvessels. 

 This corresponds suffi- 

 ciently 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 look- 

 ing at a bright and uni- 

 formly illuminated ground, 

 like the milk glass shade 

 of a lamp or the blue sky, 

 and moving the slightly 

 separated fingers or a 

 perforated card rapidly Fig- 442. Method of rendering the Retinal Blood- 

 before the eve From the vessels 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 the 

 light is shifted so as to be focussed at b, the 

 shadow cast at b' is referred to b" i.e., it appears 

 to move in the same direction as the illuminated 

 point of the sclerotic. 



rate of their apparent 

 movement, Vierordt cal- 

 culated the velocity of the 

 blood in the retinal capil- 

 laries at 0*5 to 0-9 mm. 

 per second. One reason 

 why the shadows of these 

 intra-retinal structures do 

 not appear in ordinary 

 vision seems to be their small size. The retinal vessels are in reality 

 only vascular threads; the thickest branch of the central vein is not 

 A, mm. in diameter. The apex of the cone of complete shadow 

 (umbra) cast by a disc of this size, at a distance of 20 mm. from a pupil 

 4 mm. wide, would lie only \ mm. behind the disc that is to say, the 

 umbra of the retinal vessels would not reach the layer of the rods and 

 cones at all, and only the penumbra, or region of relative darkness, 

 would fall upon it. 



When the eyes, after being closed for some time, are suddenly opened, 

 the branches of the retinal vessels may be seen for a moment. This is 

 especially the case after sleep; and a good view of the phenomenon 

 may be obtained by looking at a white pillow or the ceiling immediately 

 on awaking. If the eyes are kept open for a few seconds, the branch- 

 ing pattern fades away; if they are only allowed to remain open for 

 an instant, it may be seen many times in succession. The main vessels 

 appear to radiate out from a central point. But their actual junction 

 there is not seen, since it lies in the optic disc or blind spot. 



