478 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
I have endeavoured to identify the fixed lines in my map with the fixed lines repre- 
sented in M. Silbermann’s map of the chemical s})ectrum, with a copy of which my 
friend Professor Thomson has kindly furnished me. I am still uncertain respecting 
the identification. M. Silbermann’s map is so very much more detailed than my 
own, and must have been made with so much purer a spectrum, that the two systems 
of lines are not directly comparable. 
23. From the difficulty of identification some persons might be disposed to imagine 
that the chemical rays, and those which produced the blue light in a solution of 
quinine, were of a different nature, and had each a system of fixed lines of its own. 
For my own part, I was too well acquainted with the Protean character of fixed lines 
to regard the difficulty of identification as any valid argument in support of such a 
view. And that this difficulty arose from nothing more than the different degrees of 
purity of the spectra is now put past dispute, for my friend Mr. Kingsley of Sidney 
Sussex College, to whom I recently showed some of the experiments mentioned in this 
paper, has kindly taken for me some photographs of spectra having nearly the same 
degree of extent and purity as those with which I worked, and these show the fixed 
lines just as they appeared in a solution of sulphate of quinine and in other media*. 
24. The position of a point in the spectrum which does not coincide with one of 
the principal fixed lines, will be denoted by referring it to two of those lines, in a 
manner which will be most easily explained by an example. Thus ^GH, G|^H, GH^ 
will be used to denote respectively a point situated at a distance below G equal to 
half the interval from G to H, a point midway between G and H, and a point situated 
at the same distance above H. In using this notation, the letters denoting fixed 
lines w'ill be written in the order of refrangibility, and the fraction expressing the 
part of the interval between these lines, which must be conceived to be measured off" 
in order to reach the point whose position it is required to express, will be written 
before, between, or after the letters, according as the measurement is to be taken from 
the first line in the negative direction, from the first line in the positive direction, or 
from the second line in the positive direction, the positive direction being that of 
increasing refrangibility. 
25. From the experiments already described, it appears that the beam of dispersed 
light which was observed in the experiments of Sir David Brewster consisted of two 
very distinct portions, one arising merely from light reflected from motes, and the 
other having a far more remarkable origin. It will be convenient to have qurnes for 
these two kinds of dispersion, and I shall accordingly call them respectively false 
internal dispersion and true internal dispersion, or simply false dispersion and true 
dispersion when the context sufficiently shows that internal dispersion is spoken of. 
When dispersion is mentioned without qualification, it is to be understood of true 
dispersion. Now that it appears that the mere reflexion of light from solid particles 
held in mechanical suspension has nothing to do with that remarkable kind of internal 
* See note A at the end. 
