THE RETINA. 165 



more, and provided that they are looked at directly, that is, that their image 

 falls on the fovea. Construction by the schematic eye shows that an angle 

 of this size represents an image of about .0035 mm. on the retina, approximately 

 the distance from the centre of one cone to that of another. If the points are 

 looked at, not directly, but with 'the tail of one's eye', they must be much more 

 widely separated than this to appear distince from each other; the distance 

 necessary increases, broadly speaking, the farther they are removed towards the 

 periphery of the visual field. 



That the cells which are most deeply placed of all should be 

 those which respond to the vibrations of the light waves would at 

 first sight appear improbable. There is abundant evidence, how- 

 ever, that this is the case, as the two following experiments show. 

 Purkinje Figures. The retinal blood vessels, being situated 

 between the source of light and the receptive surface cast shadows 

 on the retina. These are very small and by habit are disregarded. 

 If, however, they can be made to fall on some part of the retina 

 which ordinarily does not receive them they may be seen. 

 Experiment 64. Make the subject turn one eye inward and look 

 towards a dark wall. With a lens concentrate a good light upon 

 the exposed sclerotic, focussing so as to make a small, strongly 

 illuminated area. Now give the lens a circular or gently rocking 

 motion. The field appears to the subject to be reddish yellow, 

 with dark branching figures on it. These are the shadows of 

 blood vessels. In the direct line of vision a small spot free from 

 shadows may be seen, the macula lutea or yellow spot. 

 The path of the light rays through sclerotic and choroid and 

 the vitreous humour is shown in the diagram (Fig. 47). When 

 the source of light is moved there is a movement of the shadow of 

 the blood vessel on the retina, which is projected outward by the 

 subject along the line which connects the point stimulated with the 

 nodal point, and which is interpreted as a movement of a shadow in 

 the field of vision at the distance for which the eye is accommodated. 

 A is the first position of the light, A' the place on the retina where 

 it throws the shadow, A" the place on the screen at which the eye 

 appears to see the shadow. When the light is moved to B the 

 shadow moves to B' and its apparent place in the field to B". 

 n.p. is the nodal point of the eye, b.v. the position of the blood 

 vessel. Now triangles A"B" n.p. and n.p.A'B' are similar and we 

 know the lengths A"B", n.p.A" and n.p.A'. We can therefore find 



