HELMHOLTZ IN BERLIN 



ness was limited by the interval between the glasses. 

 Of course the central part of each crystal, except the 

 smallest ones, was bounded by parallel planes, but the 

 edges were bevilled at various angles, forming so many 

 little prisms, the smallest of them floating in the liquid. 

 When a distant candle was viewed through these 

 glasses, having the little prisms interposed, a great 

 number of spectra became visible, caused by the in- 

 clined edges. Most of these were no doubt very 

 imperfect, but by trying the glass at various points, 

 some very distinct spectra were met with, and these 

 could, with some trouble, be isolated by covering the 

 glass with a card pierced with a pin-hole. It was 

 then seen that each prism (or oblique edge of the 

 crystal) produced two spectra oppositely polarised and 

 widely separated. One of these spectra was normal ; 

 there was nothing particular about it. The colours of 

 the other were very anomalous^ and, after many experi- 

 ments, I came to the conclusion that they could only 

 be explained by the supposition that the spectrum, 

 after proceeding for a certain distance, stopped short and 

 returned upon itself? 



The words italicised show that Fox Talbot dis- 

 covered the real nature of this curious phenomenon. 

 Le Roux, in 1860, had found that vapour of 

 iodine, which allows only red and blue rays to pass, 

 refracts the red more than the blue, the opposite of 

 the effect of a glass prism. An alcoholic solution of 

 fuchsine (an aniline colour) gives a dark absorption 

 229 



