116 SIR DAVID BREWSTER ON THE DECOMPOSITION AND DISPERSION OF LIGHT 
dispersion, provided the thickness of the medium is not so great as to have dis- 
persed all the dispersible rays. 
2. When such a medium is thus rendered incapable of dispersing more light, 
it is not because it has lost a property which it originally possessed, but because 
it is deprived of all the dispersible rays which it contained. 
It is no doubt an interesting fact, that a small number of differently coloured 
rays, constituting blue light by their mixture, should possess this property of 
being dispersed, while other rays of the same refrangibility are either less disper- 
sible, or apparently indispersible, by the same medium ; but the fact will appear 
less surprising and anomalous when we advert to certain phenomena of absorp- 
tion in which the same property is displayed. 
The difference between the absorption and the internal dispersion of light is 
simply this. In the one case the portion of light withdrawn from the intromitted 
beam is extinguished and invisible, and in the other dispersed and visible ; and we 
may compare the two classes of phenomena by supposing that the light extin- 
guished by absorption is rendered visible as if by dispersion. Now it is a remark- 
able fact, that almost the whole of the blue light absorbed by the mineral called 
native orpiment is extinguished during the passage of the light through the first 
stratum, whose thickness is less than the fiftieth of an inch; and hence it is that 
the thinnest slice of this substance has nearly as deep a yellow colour as the 
thickest. Were the absorbed blue rays to become visible by dispersion, we should 
actually see a more striking example of epipolism, or dispersion confined to the 
first stratum, than in the quiniferous solution. Even the condensation of the 
beam would not in this case give us a blue cone of light. 
The analysis of the blue line indeed would indicate a difference between the 
two phenomena. It would shew that the blue light was derived chiefly from the 
violet, indigo, and blue spaces, and but partially from the green, yellow, orange, and 
red, having appropriated the whole of the more refrangible rays, and but a very 
small portion of the less refrangible ones; whereas the blue light from the quini- 
ferous solution is derived almost in equal proportions from all the coloured spaces 
excepting the least refrangible, red. The limitation of the rays capable of ab- 
sorption, like the limitation of the dispersible rays in the quiniferous solution, is 
shewn in the action of various bodies on the spectrum. Such bodies change the 
colour of certain spaces in the spectrum, without continuing to absorb the resi- 
dual rays; that is, when the absorbable rays are removed by a certain thickness 
of the body, an additional thickness operates very feebly, as in the quiniferous so- 
lution, in altering the colour of the residual beam. 
I have pointed out these analogies between the phenomena of absorption 
and dispersion to meet the case of the bright blue line in the quiniferous solu- 
tions. The dispersion of fluor-spar, and of the glasses and vegetable solutions 
already described, is of a different character. In fluor-spar the dispersion effected 
