ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



245 



blinking on the negative afterimage seen on the white surface. It 

 will become during the shutting of the eyes converted into a positive 

 after-image.] 



FIG. 215. Disc for the experiment on after-images of motion. 



[ADVANCED EXPERIMENT. 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.] 



[ADVANCED EXPERIMENT. After-images of motion may be shown 

 by gazing at the disc in Fig. 215 slowly rotated and then shifting 

 the gaze to some uniformly mottled surface.] 



CHAPTER LXIX. 

 SENSATIONS OF LIGHT AND COLOUR. 



MANY theories have been advanced to explain the phenomena con- 

 nected with colour vision. The most important of these theories are 

 those connected with the names of Young-Helmholtz and Hering. 

 The theories are all concerned in referring the multiplicity of colour 



