PURKINJE'S SHADOWS 711 



appear. This happens when its image falls on the retina directly over the 

 entrance of the optic nerve, which has no visual cells, and is, therefore, the 

 blind spot. This area is large enough to cause a man completely to disappear 

 at a distance of about one hundred meters. 



Place a sheet of white paper at a distance of 30 cm. in front of the eye, 

 holding the head in a fixed position against the special support furnished; 

 look with the right eye at the top of the cross made on the left of the sheet 

 of paper. Covering the sharpened portion of a lead pencil with white 

 paper, leaving the black tip exposed, move this pencil across the paper 

 from the visual center to the right. At a certain distance the black lead 

 will suddenly disappear. 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 computation 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, b, 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, 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 determined. This is especially true if there is slight retinal 

 congestion. 



