HELMHOLTZ IN KONIGSBERG 



them added to a certain amount of white. Thus 

 100 parts of green are composed of 15 of #, 51 of />, 

 and 34 of c ; or, to take other examples, spectral red 

 contains, in 100 parts, 42 of #, I of , and 57 of white ; 

 yellow ii of a, 14 of , and 75 of white ; and blue 

 2 of a, ii of f, and 87 of white. The white gives 

 the element of brightness. According to this view, 

 it is not necessary to suppose that in the red- 

 blind the red-perceiving elements are awanting, or 

 that in the green-blind the green-perceiving elements 

 are absent, but that these elements may be stimulated 

 with intensities different from those affecting the 

 normal eye. 1 



It is foreign to the purpose of this book to criticise 

 the theory, or to contrast it with the rival theory of 

 Hering, which assumes molecular processes in the 

 retina of a katabolic (pulling down or disintegrative) 

 and an anabolic (building up or reconstructive) kind. 

 Suffice it to say, that while there are a few special 

 cases not yet completely explained by the Young- 

 Helmholtz theory, on the whole it accounts for 

 the general facts in a satisfactory and convincing 

 manner. 



The investigation is eminently characteristic of 

 Helmholtz. He examined the facts with the min- 

 utest care, and with the aid of arrangements and 

 apparatus of the most ingenious and perfect kind, and 

 then he endeavoured to refer all the facts to a general 



1 M'Kendrick and Snodgrass, of. cit., p. 169. 

 I2 7 



