ELEMENTARY EXPERIMENTAL PHYSIOLOGY 111 



red. Again look at the filament but for a prolonged period of about 

 half a minute. On closing the eyes the after-image will appear bright 

 but greenish in colour. 



By an alteration of light and dark backgrounds the after-image may 

 be changed from negative to positive. 



EXPERIMENT III. Look at an incandescent lamp for half a minute 

 and so get a well marked after-image. If the eyes be directed to a white 

 surface the after-image will be negative, if to a dark surface it will 

 appear positive. 



EXPERIMENT IV. Note the colour of the after-images in Experiment 

 III., and the gradual change in colour which they show. If the after- 

 images tend to fade blink the eyes several times rapidly and they will 

 become more marked. Notice especially the effect of blinking on the 

 negative after-image seen on the white surface. It will become during 

 the shutting of the eyes converted into a positive after-image. 



unifor 



FIG. 96. Disc for the experiment on after-images of motion. 



EXPERIMENT V. Look at an incandescent lamp with the right eye, 

 the left eye being closed. After the lapse of half a minute, shut the 

 right eye and look with the left at a dot on a white sheet of paper, as 

 far as possible without blinking. After a time the field will gradually 

 darken and a positive after-image will be seen. This is really the after- 

 image seen with the right eye, which is not visible till a certain amount 

 of retinal insensibility has occurred in the left eye. 



EXPERIMENT VI. After-images of motion may be ^shown by gazing 

 at the disc in Fig. 96 slowly rotated and then shifting the gaze to some 

 uniformly mottled surface. 



