410 J\I. H. Helmholtz on Sir David Brewster's 



By this method we can obtain bright a band of any breadth 

 whatever, if instead of the first sht a rectangular opening of 

 greater or less width is cut out. The spectrum of the first pi'ism 

 will then be an impure one ; that is, at every point of it the 

 neighbouring bands of colour will overlap each other to a certain 

 extent ; hence regularly refracted light of different kinds passes 

 through the slit and is decomposed by the second prism into its 

 component toues of colour. In this way is obtained a more or 

 less bright, sharply-defined band, furnished with its appropriate 

 lines of Frauuhofer, and composed of those overlapping colours 

 which fell from the first prism upon the slit ; while the remaining 

 portion of the second spectrum, illuminated merely by the di- 

 spersed light, remains very feebly luminous. In this way the 

 violet at the other side of the line H, which when the other 

 colours are ])resent is usually regarded as invisible, can be ren- 

 dered surprisingly distinct, being obtained free from white light 

 for a width equal to that between the hues G and H. When 

 viewed in the ordinary way through a telescope, the remaining 

 portion of the spectrum being shut out, it is usually mixed with 

 an inordinate quantity of white light. 



If we isolate the light of the yellow band of the smalt-glass 

 spectrum according to this method, and subject it to the 

 absorption of a certain number of plates of the glass introduced 

 before the first or the second slit, or before the eye, we obtain 

 results totally different from those arrived at by the method of 

 Brewster. The yellow retains its originally pure and saturated 

 colour after it has passed through two, three, or even four plates 

 of the blue glass. I may further remark, that an absolutely 

 dark room is not essential to the success of this experiment, if 

 care be taken that the second screen is sufficiently black, and 

 the plates of glass are placed before the first slit. 



Brewster's explanation is irreconcilcable with this observation. 

 According to his view, the light of the yellow band, when ren- 

 dered whitish by the cobalt-glass, is composed of rays of equal 

 refrangibility, and hence by refraction in prisms cannot be further 

 decomposed into rays of different colours. In the experiment 

 above described, the light of the yellow band on entering the first 

 slit appears actually whitish ; but when it is viewed through a 

 second prism, it is decomposed into pure yellow and light of 

 other colours; hence it does not possess the same refrangi- 

 bility, but, in accordance with the explanation given by me, is 

 a mixture of rays of difi"erent refrangibilities. In Brewstei-'s 

 proceeding, a mixture of foi'eign light, whether in the pi'ism, 

 glass plates or eye, could not be avoided. Fi'om this point of 

 view it is quite intelligible how the introduction of coloured 

 media might render the white colour of the band in question 

 more pure, or cause it to approximate to red or green. 



