7 6 4 



THE SENSES 



the limits of the blind spot in the horizontal plane, as magnified by the 

 conditions of the experiment. Mark the limits in the other meridians in 

 the same manner. Compute from the figures obtained the exact size of 



FIG. 485. The Blind Spot with the Eye 30 cm. from the Paper. The irregularity of 

 outline is due to the larger blood vessels. 



the blind spot in your right eye, figure 485. Repeat on the left eye. Usually 

 these areas are not symmetrical. The computation may be based on the 

 following proportion: a, the distance of the map from the nodal point of the 

 eye, is to b, the diameter of the map of the blind spot, as c, the distance from 

 the nodal point to the retina, which is i. 5 cm., is to x, the diameter of the 

 actual blind spot in the retina, a : b : : c : x. x varies from i . 5 to 3 or more 

 mm. 



19. Relations of the Size of the Retinal Image to Distance. Com- 

 pute the size of the retinal images of familiar objects by the equation given 

 in the last experiment. Compute the size of the image formed on the retina 

 by a man six feet tall at a distance of 100 feet. Compute the size of the 

 image formed by a tower 125 feet tall at a distance of 575 feet. 



20. Purkinje's Shadows. Stand in front of a blackened wall in 

 the dark room. While looking toward the wall with the right eye accom- 

 modated for distant objects, move a lighted candle back and forth about 

 10 to 20 cm. to the right of the eye and a little below its level. Presently 

 many branching shadows will be seen as though they stood in space in front 

 of the individual. These are the shadows of the blood vessels cast upon the 

 retina. A careful examination will show that these shadows seem to con- 

 verge to a point to the right of the center of vision of the right eye. By 

 moving the candle up and down or from side to side, the shadows seem also to 

 move slightly. Many persons can readily see Purkinje's figures by looking 

 through the narrow spaces between the fingers of the hand moved close in 

 front of the eye, when the vision is directed toward a bright sky. One can 

 demonstrate by this means that the macula is free from blood vessels, since 



