PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 527 
Of these the latter would be insignificant compared with the former, were it not 
that the illuminating power of the colours belonging to the middle of the spectrum 
is so very much greater than that of the violet. When the dispersed beam was 
analysed by a prism, it would be decomposed into a violet beam of definite refrangi- 
bility, followed by a dark interval, and then a broad band containing the colours of 
the brighter part of the spectrum in their natural order. This is what is constantly 
seen in cases of true dispersion; but the polarization of the beam, and its behaviour 
under the action of absorbing media, would reveal the counterfeit character of the 
dispersion. 
On the Colours of Natural Bodies. 
1/4. By this expression I mean to include only the colours to which it is usually 
applied, namely, those of leaves, flowers, paints, dyed articles, &c., which form the 
great mass of the colours that fall under our observation. I do not refer to colours 
due to refraction, such as those of the rainbow, or to diffraction, such as those of the 
coronee seen about the sun and moon, or to interference, such as those seen in the 
clear wings of small flies, or to the colours which accompany specular reflexion, 
which last are usually but slight, though sometimes pretty intense. 
In some few instances, as for example in the case of fluor-spar, various salts of 
peroxide of uranium, acid solutions of disulphate of quinine, &c., colours are observed, 
sufficiently strong to arrest attention, which have a remarkable and hitherto unsus- 
pected origin. But I am not now speaking of colours arising from a change of 
refrangibility in the incident light. In the vast majority of cases these colours are 
far too feeble to form any sensible portion of the whole colour observed. The colours 
which dyed articles give out under the influence of the highly refrangible rays usually 
agree more or less nearly with those of which such substances commonly appear, 
and it is possible that the colour arising from a change of refrangibility may contri- 
bute in some slight degree to the brilliancy of the tint observed. If, however, the 
effect be sensible I am persuaded that it is but slight ; and very brilliant colours may 
be produced without a change of refrangibility, as for example in the case of biniodide 
of mercury. For the present I shall neglect the light which may have changed its 
refrangibility. 
175 . Few, I suppose, now attaeh much importance to the bold speculations in 
which Newton attributed the colours of natural bodies to the reflexion of light from 
thin plates. Sir David Brewster has shown how extremely different the prismatic 
composition of the green of the vegetable world is, from what it ought to be, accord- 
ing to Newton’s theory, and what Newton supposed that it was. It is now admitted 
that the various colours of natural bodies are merely particular instances of one 
general phenomenon, namely, that of absorption. Absorption is most conveniently 
studied in a clear fluid or solid, but it does not the less exist in a body of irregular 
structure, such as a dyed cloth or a coloured powder. 
MDCccLii. 3 Y 
