SENSE OF SIGHT. 577 



way: Look with the right eye at the round black spot here 

 printed, 



closing the left eye, and holding the book six inches from it. The 

 spot and the cross can both be seen. Now carry the book away 

 from the face farther and farther, still looking at the spot. A 

 point will be reached where the cross will at once disappear, and 

 when this occurs the light from the cross falls upon the optic disk. 

 If the book is carried still farther, the cross will again come in 

 sight. 



There is no doubt that the portion of the retina which reacts 

 to the stimulus of light is the layer of rods and cones, and of this 

 layer the cones are especially sensitive. This is shown by the 

 fact that the macula lutea (yellow spot) is the portion of the retina 

 which is the most sensitive, and here there are no nerve-libers, but 

 rods and cones, and in the fovea centralis, which is the most sensi- 

 tive portion of the macula, only cones are found. 



Purkinje's Figures. It may also be shown by the following 

 experiments : 1. If in a dark room a small candle-flame is moved 

 to and fro, close to and at the side of the eye, while the latter is 

 directed toward the dark, an outline of the blood-vessels of the 

 retina will be seen. 2. Or, if after the eyes have been closed for 



FIG. 356. Method of rendering the retinal blood-vessels 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, r, is cast at a'. It is referred to an ex- 

 ternal 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 (Stewart). 



some time, as upon awaking from sleep, the eyes are directed for 

 an instant to a white ceiling, an outline of the retinal blood-vessels 



37 



