HELMHOLTZ IN KONIGSBERG 



to a series of waves differing in rate of succession from 

 the others. Thus the waves that give rise to a sensa- 

 tion of red light follow each other at the rate of about 

 435 millions of millions per second, while those of 

 violet light succeed each other at about 764 millions 

 of millions per second. Between these we have an 

 infinite number of series of waves, each giving rise 

 to a special colour sensation, and so between the red 

 and the violet of the spectrum we have a gradation of 

 colour roughly described as orange, green, blue and 

 indigo, but each of these is itself made up of countless 

 shades, which melt as gradually and imperceptibly 

 into one another as the colours of a sunset sky. The 

 retina is not sensitive to vibrations of the ether suc- 

 ceeding each other more slowly than those of red 

 light, although it may be demonstrated that these 

 exist and originate electrical and thermal phenomena ; 

 nor to those which come more quickly, although the 

 latter have marked chemical activity, and give rise 

 to fluorescence. 



Solar or white light is, then, a compound of all the 

 colours in definite proportion. A body which reflects 

 solar light to the eye without changing this propor- 

 tion appears to be white ; but if it absorbs all the 

 light, so as to reflect no light to the eye, it appears to 

 be black. If a body held between the eye and the 

 sun transmits light unchanged and is transparent, it 

 is colourless ; but if translucent, it is white. If it 

 transmits or reflects some rays and absorbs others, it 



