THE EYE AND THE SENSE OF SIGHT 125 



62. Duration of Sight Sensations. Cause a small wheel to revolve 

 rapidly ; the spokes no longer appear distinct, but seem to be thinned 

 out and fused together into a semitrausparent membrane. Spin a 

 top composed of an angular piece of card fastened to a suitable axis. 

 It appears circular instead of angular. 



63. Look at a bright light for a moment. Then close the eyes; 

 the image persists for a short time. 



64. Fatigue of the Retina. After Images. Look steadily, for one or 

 two minutes, at a window, and then at a plain light-colored wall. An 

 image in which the light parts of the window are dark and the dark 

 parts are light will now be seen. 



65. Complementary Colors. Look steadily at a piece of red paper 

 and then at a light background ; a light green after image will be 

 seen. Repeat, using different colored papers. From dealers in kin- 

 dergarten supplies can be obtained packages of colored papers suitable 

 for this and other experiments on color. 



66. Color Blindness. The teacher frequently finds pupils unable to 

 name colors correctly. This may be due to color blindness, but in 

 most cases is caused by defective training. To make any accurate 

 tests of color blindness, the teacher should procure a set of Holm- 

 gren's test worsteds (price $2.50) and directions for experimenting. 



