HERMANN VON HELMHOLTZ 



is coloured. If, for example, it absorbs all the rays of 

 the solar spectrum but those which give rise to the 

 sensation of greenness, we say that the body is green 

 in colour. But this greenness can only be perceived 

 if the rays of light falling on the body contain rays 

 which have the special vibratory rate that is required 

 for this special colour. For, if we use as our light 

 any other pure-coloured ray of the spectrum, say the 

 red, its rays being absorbed, the body appears to us to 

 be black. A white surface seen in a red light seems to 

 be red, in a green light green, as it reflects all colours 

 alike, absorbing none. To the normal eye the colour 

 physically depends, then, on the nature of the surface 

 of the body, as was first shown by Robert Boyle, 

 and of the light falling upon it, and the sensation of 

 colour only arises when the body reflects or transmits 

 the special rays to the eye. If two rays of different 

 wave-lengths affect one part of the retina at the same 

 time, they are fused together, and we have the sensa- 

 tion of a third colour different from its cause. Thus, 

 if red be removed from the solar spectrum, all the 

 others combined will give a sensation of a greenish 

 yellow, although we cannot, with the unaided eye, 

 analyse this into its components. Certain colour 

 sensations, such as red or green, are simple in the 

 sense that they cannot be originated by any combina- 

 tions of other colours ; while other colours, such as 

 purple, can be produced by certain definite mixtures, 

 and they are therefore called compound. 

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