124 PHYSIOLOGY FOB THE LABORATORY. 



V 



N. After-images. 



I. POSITIVE AFTEK-IMAGES. 



Apparatus. A gas-jet with a white globe. 



Rest the eyes by closing them, and then suddenly open 

 the eyes and look for a few seconds at the gas-jet. Now 

 close the eyes again and notice what you see. 



II. NEGATIVE AFTER-IMAGES. 



Material. Papers of different colors, as red and blue. 



Experiment : 



1. Look steadily for half a minute or longer at a 



small white square on a black ground. Then 

 slip a piece of white paper over the whole 

 and notice the image. 



2. Look steadily at a bright red square on a black 



ground, and then slip a piece of white paper 

 over them. What color is the after-image? 



3. Obtain the after-images with other colored papers. 



These images are due to fatigue of the retina 

 and brain. What relation do the colors of 

 the after-images bear to the colors of the 

 paper. 



