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EXPERIMENTAL PHYSIOLOGY. 



A'B', the distance the shadow actually moved on the retina. But 

 triangles A'B' b.v. and bv.AB are also similar. Here we know 

 A'B', A.B. and (approximately) bvA., so that we can find bv.A', 

 the distance between light-sensitive layer and the blood vessel. 

 This is found to be about 0.2-0.3 mm., which is sufficiently like the 

 distance actually existing between the retinal blood vessels and the 

 layer of rods and cones to support the conclusion that these are 

 the elements sensitive to light. 



FIG. 47. To illustrate the course of the rays w hich cast the shadows in Purkinje's 

 figures, and the direction along which these shadows are projected outward into the 

 visual field. 



Blind Spot. It may easily be shown that, although we are 

 unconscious of it, there is an area in the field of vision of each eye 

 which is blank; objects placed there are not seen. Construction 

 with the schematic eye shows that light rays from this area fall on 

 the optic disc. Here, as we have seen, there are no rods or cones, 

 the layers which contain them being interrupted to allow of the 

 passage through it of the fibres of the optic nerve. 



