576 STUDIES IN ADVANCED PHYSIOLOGY. 



Hering explains monochromatic eyes as eyes that pos- 

 sess but this white-black substance. He questions the 

 bluish tint which many monochromatics have and explains 

 the appearance of the world as monochromatics see it as 

 that of a black and white steel engraving. As this white- 

 black substance is produced when no light enters the eye 

 things seem black in the absence of light. 



Dichromatics possess in addition to this white-black 

 substance a blue-yellow substance. This substance when 

 it is being destroyed in the eye produces blue ; when it is 

 being re-formed in the eye produces yellow sensations. 

 Looking at a blue object according to this theory would 

 produce a disintegration of this substance, which would 

 occasion the sensation blue, while looking at a yellow ob- 

 ject would cause the production of some of this substance 

 and in this way the retina be affected to see yellow. 



Finally, in normal eyes there is, in addition to the 

 white-black substance and the blue-yellow substance, a red- 

 green substance. Here, too, a disintegration of the same 

 produces red, a production of it, green sensations. While 

 this theory seems very fanciful at first, it explains the 

 dichromatic eyes and further gives us a reason for many of 

 the phenomena which may be observed in studying after- 

 images and color contrasts. 



AFTER-IMAGES. 



An after-image is, as the name suggests, an image re- 

 tained on the retina after the stimulus producing it has 

 ended. Such after-images may be of two kinds: positive, 

 when they have the same color as the object, or negative, 

 when they have the complementary color. 



1. Positive After- Images. The forming and dis- 

 appearance of an image on the retina is, from a physical 

 standpoint, as instantaneous as the changes of light, but 

 the sensation which this image produces will sometimes 

 continue after the stimulus is gone. Thus, a sky rocket 

 seems to leave a trail of fire after it ; a red-hot coal swung 



