414 M. H. Helmholtz on Sir David Brewster's 



This is most easily observed with violet, which, in the spectrum 

 of dii-ect sunlight and by a moderate degree of brightness, appeal's 

 to be a white-gray, retaining only a feeble tinge of violet. Pro- 

 fessor ]\Ioser has shown me that the sun seen through a dark 

 violet glass appears as completely white as the strongly illumi- 

 nated clouds observed on looking past the glass. In like manner 

 blue, of a degree of brightness which may be borne without 

 injury to the eye, appears whitish-blue, and, if the brightness be 

 increased, appears white. Green first becomes yellow-greeu 

 before, like yellow, it entirely loses its colour with increased 

 brightness. Red exhibits the ph?enomenon with more difficulty 

 than all others ; and only by the highest degree of brightness 

 have I been able to see it bright yellow in the spectrum, or the 

 sun of the same colour when looked at through a red glass. 

 While experimenting on this subject, in order to prevent the 

 admixture of all light of other colours, I have made use of the 

 coloured bands of the solar spectrum which were separated and 

 purified by two prisms in the manner already described. Dif- 

 ferent degrees of brightness were obtained by applying the 

 light of brightly illuminated portions of the firmament; but 

 as, according to Brewster's theory, the colours of the spectrum 

 produced by sunlight are not the same as those produced 

 by the light of the firmament, I also made use of the direct 

 solar rays, sometimes observing them directly, sometimes trans- 

 mitting them through two Nichol's prisms crossed perpendicu- 

 larly. By reflexion from imcoated glass plates, or by receiving 

 them upon a white screen, the brightness can be dimmed without 

 fear of altering any colour. 



If, therefore, a certain thickness of the solution of ammonio- 

 sulphate of copper shows the blue of the spectrum bi-ight and 

 whitish, while a greater thickness causes it to appear a deep 

 dark blue, we are simply to conclude that this fluid absorbs blue 

 rays, but by no means that it has abstracted white light from 

 the blue. Herein also we find the explanation of the fact, that 

 the yellow in the spectrum of daylight or of the blue firmament 

 is scarcely discernible, while in the spectrum of direct sunlight 

 it takes up a wide space. The pure yellow forms in a flint-glass 

 spectrum an extremely narrow band, and with the blue light of 

 the firmament is weaker than its neighbouring colours, so that 

 in the slightly magnified spectrum it is hardly discernible between 

 the wide and shining red and green. When considerably mag- 

 nified, on the contrary, or when viewed isolated, it is very plain 

 even with daylight. In the spectrum of direct sunlight, how- 

 ever, yellow is tlic most prominent colour and of dazzling bril- 

 liancy. Green and red, with increased intensity, become also 

 yellowish, and hence it is that yellow appears so prominent. If, 



