NEUEO-MUSCULAE MECHANISM 



What we call colours are particular changes in our conscious- 

 ness which accompany particular changes in our brain neurons 

 produced by particular changes set up in the retina, in what- 

 ever way these changes may have been produced. 



4. After looking for some time at any one colour, on remov- 

 ing the colour another appears in its place the eomplemental 

 colour. If the first colour is 



Eed, the second will be green blue ; 

 Orange, blue; 

 Green, pink ; 



Yellow, ., indigo blue ; 



and vice versa. (Practical Physiology. ) 



Theories of Colour Vision. 1. From consideration of the per- 

 ipheral colour-blind zone 

 of the retina and of the 

 more limited area giving 

 sensations only of blue 

 and yellow when stimu- 

 lated, and of the most 

 limited central part giv- 

 ing also sensations of 

 red and green, it would 

 seem that some special 

 substance or substances 



must exist in each of FlG - 63< 



these areas which by its or their stimulation give rise to the 

 various sensations. 



2. Considered along with this, the phenomena of eomple- 

 mental colours suggest the possibility of there being one 

 substance which when undergoing one change, say breaking 

 down, produces yellow, and when undergoing another change, 

 say building up, produces blue, and another substance which 

 when undergoing one change produces red, and another change 

 produces green. If such a view be correct, it becomes almost 

 necessary to postulate the existence of another substance 

 which when stimulated gives rise to sensations which we 

 call white. Or there might be four different substances, one 

 when changed giving rise to yellow, another to blue, another 

 to red, and another to green. When the substance giving 



