106 



HABIT AND INTELjuIGENCE. 



[chap. 



to be contradicted by the well-known fact that presMu-e on the 

 retina produces the sensation, not of pressure, but of light. But 

 when the retina and the optic nerve have been habitually trans- 

 Sensations mitting sensations of light ever since their first formation, it is 

 I think what might have been expected that all their sensations, 

 even when excited, not by radiance, but by pressure or by 

 disease, should take the form of sensations of light. 



It is stated, that an electric shock sent in opposite directions 

 along the optic nerve will produce, if in the one direction the 

 sensation of red, and if in the other direction the sensation of 

 blue.^ This is a strong argument against the hypothesis of 

 distinct nerves for each of the three primary colours ; for it is 

 difficult to believe that the set of nerves which produces the 

 sensation of blue will produce their sensation in response to an 

 electric current in one direction, that the set which produces 

 the sensation of red will respond to a current in the opposite 

 direction, and that the set which produces the sensation of 

 green will respond to neither. It is far more likely that the 

 opposite electric currents, or shocks, produce different sensations 

 in the same nerves. 



of light 

 due to 

 pressure, 



and to an 



electiic 



current. 



NOTE B. 



COLOURS AND THE LAWS OF THEIR COMBINATION. 



Difference 

 between 

 sensations 

 of sight 

 and of the 

 other 



Meaning 

 of light 

 and of 

 radiance. 



Heating 

 and 



chemical 

 effects of 

 radiance. 



There are a great many distinct primary kinds of sensation of 

 taste, of smell, and of sound. The same might appear to be 

 true of the sensations of colour; but such is not the fact j all 

 the sensations of colour which are possible, at least to the human 

 eye, are capable of being produced by the combination of three 

 primary elements. 



In what follows, I shall use the word light as only the name 

 of a sensation. That which produces the sensation I shall call 

 radiance. Eadiance, however, has other properties than that of 

 producing the sensation of light when it falls on the retina. It 

 is a form of energy, and, like all forms of energy, it is capable 

 of being transformed into heat : tliis takes place when sunbeams 

 heat the body on which they fall. It has also many peculiar 

 and little understood chemical effects ; the whole art of photo- 

 graphy is based on some of these. 



1 I regret that I cannot find my authority for this statement. 



