HERMANN VON HELMHOLTZ 



narrower than the other and blacker, while the 

 broader band is more towards the right, that is, nearer 

 E. The remaining part of the spectrum shows all 

 the colours with beautiful clearness. The analysis of 

 light by means of the spectroscope affords, then, a 

 ready means of examining the phenomena of absorp- 

 tion, an example of which has already been given in 

 treating of Helmholtz's explanation of the fact that 

 the mixture of a yellow with a blue pigment produces 

 green. 



The peculiar phenomenon of anomalous disper- 

 sion is closely associated with that of absorption. 

 It was first discovered by Fox Talbot about 1870.* 

 The experiment of Fox Talbot was as follows : ' I 

 prepared some square pieces of window glass, about an 

 inch square. Taking one of these, I placed upon it a 

 drop of a strong solution of some salt of chromium, 

 which, if I remember rightly, was the double oxalate 

 of chromium and potash, but it may have been that 

 substance more or less modified. By placing a second 

 square of glass on the first, the drop was spread out in 

 a thin film, but it was prevented from becoming too 

 thin by four pellets of wax placed at the corners of 

 the square, which likewise served to hold the two 

 pieces of glass together. The glasses were then laid 

 aside for some hours until crystals were formed in the 

 liquid. These were necessarily thin, since their thick- 



1 Proc. Roy. Soc. Edin., 1870-71. See also Tait on Light, p. 156. 

 Edinburgh, 1884. 



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