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IV. — On the Connection between Fluorescence and Absorption. 
By H. C. Sorby, F.R.S., &c., President R.M.S. 
Though the relation between fluorescence and absorption has already 
attracted a good deal of attention, there are some important facts 
which appear to be often imperfectly known. I have been surprised 
to find that even some of those who have paid considerable attention 
to such subjects have so far misunderstood the question as to 
suppose that the light of the fluorescence consists of rays which 
are as it were reflected by the solution, and do not penetrate 
through it, so that the spectrum of the fluorescence would show a 
bright band in the same place as some dark band seen in the 
spectrum of the transmitted light. This is certainly an error. 
A much nearer approximation to the truth is the conclusion arrived 
at by Lubarsch, who has recently published a paper on this subject 
in Poggendorff’s ‘ Annalen,’* of which a good outline is given in 
‘ JDer Naturforscher,’ 1875, Feb. 6th, p. 52. It is the appearance 
of this paper which has induced me to describe some of my own 
observations. He gives a table showing the connection between the 
fluorescence and absorption of eight different substances, all of 
which show that the spectrum of the light of fluorescence extends 
some distance on the red end side of the principal absorption band 
in the spectrum of transmitted light, commencing very nearly at 
that part which corresponds to the centre of the absorption band, 
or maximum point of absorption. He expresses this law by saying 
that “ the spectrum of fluorescence of a substance can never contain 
rays which are more refrangible than those which are most readily 
absorbed by a very dilute solution.” To express this in wave-length 
phraseology, I should say that if a fluorescent solution gave by 
transmitted light a spectrum having a well-marked absorption band, 
whose centre corresponded to light of some particular wave-length, 
as, for example, 600 millionths of a millimeter, the spectrum of 
the light of fluorescence would extend from nearly that point 
towards the red end, so as to include a band of light of wave- 
lengths varying from 600 to some other greater length, according 
to each particular substance. This is certainly such a common 
fact, that for a long time I was disposed to believe it to be general, 
but I have now had the opportunity of examining a greater number 
of fluorescent substances, and find that there are very decided 
exceptions. It appears to me important to know what is the true 
state of the case ; since, as I shall show, the study of the spectra of 
fluorescence may often be of great value in deciding whether any 
solution contains a number of different substances in solution, or 
only one. 
In studying the spectra of fluorescent solutions, I often find it 
* Vol. cliii., p. 420. 
