676 THE SENSES 



Mark this point. Continue to move the pencil until the lead reappears. 

 Mark this point. These two points represent 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 the blind spot in your right eye, figure 485. 

 Repeat on the left eye. Usually these areas are not symmetrical. The com- 

 putation may be based on the following proportion: a, the diameter of the 

 mapped blind spot is to the distance of the map from the nodal point of the 

 eye, x, as c, the distance from the nodal point to the retina, which is 1.5 cm., 

 is to x, the diameter of the actual blind spot in the retina, x varies from 

 1.5 to 3 or more mm. a : b : : c : x. 



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 the pattern 

 of the blood-vessels around the borders of the macula is very readily de- 

 termined. This is especially true if there is slight retinal congestion. 



21. Duration of the Retinal Image. When a beam of light falls 

 upon the retina for an instant it produces a stimulus which endures for a 

 time after the stimulus is removed. This interval can be measured by the proper 

 mechanical device. Place on the color wheel a disc, which has a small seg- 

 ment cut out at one point on the periphery. Put a printed page behind the 

 segment with the observer standing in front. Rotate the segment faster and 

 faster until the printed page is seen continuously. At this point the visual 

 image made at one revolution of the disc lasts until the next impression on 

 the same spot. The speed of the revolution of the color wheel can be measured 

 by attaching an electric contact key and signal magnet to the disc wheel and 

 measuring the rate of interruptions against the known vibrations of a tuning 

 fork. The same phenomenon may be determined by placing on the disc 



