163 
Fluorescence and Absorption. By H. C. Sorby. 
of the centre, and be bounded by the limit of absorption towards 
the red end. We may thus easily explain why there is a slight 
difference in the case of different substances ; since, if the intensity 
of absorption be greater in proportion to the intensity of fluores- 
cence, all other circumstances being equal, the limit of the light of 
fluorescence will lie more towards the red end than the centre of 
the main absorption band, than if the absorption be relatively 
weaker. The result is that in the case of the substances I have 
examined, the average limit of the light of fluorescence lies a very 
little on the red side of the centre of the main absorption band, 
which is in exact agreement with the observations of Lubarsch, 
t ;ough there is a very decided variation in different cases not 
shown by the substances named in his published list. 
The knowledge of this law is very important in studying such 
mixtures of different colouring matters as are obtained in examining 
animals and plants. For example, when the superficial mem- 
branous coloured layer of some such species of fungi as Russula 
nitida and vesca is digested in alcohol, a purple fluorescent solution 
is obtained, which gives a spectrum with a well-defined absorption 
band, having its centre at wave-length 554 millionths of a milli- 
meter, whilst the spectrum of fluorescence extends far beyond that 
towards the blue end, up to wave-length 440. This led me to 
conclude that the coloured solution was a mixture of a purple non- 
fluorescent substance with one that is pale yellow and fluorescent ; 
and after trying various methods, I at length succeeded in separating 
two substances of exactly that character. If it had not been for 
such theoretical considerations, I might never have suspected that 
there was any such mixture. Another admirable illustration is 
furnished by the beautiful purple fluorescent solution obtained by 
keeping Oscillatorise in water, described by me in this Journal, vol. 
iii., p. 229. This gives a spectrum with two well-marked absorp- 
tion bands, having their centres at wave-lengths 620 and 569. 
The spectrum of fluorescence shows two bright bands, having their 
centres at 647 and 580, and their limits towards the blue end at 
632 and 57 1 , which bright bands are thus manifestly related to the 
absorption bands, as though they were due to two independent 
fluorescent substances mixed together. Now by heating this solu- 
tion to the point of the coagulation of albumen, a pink substance is 
deposited, and the clear solution gives a spectrum with only the 
band at 620, thus proving that the original solution is a mixture — 
a conclusion borne out by many other facts. In a similar manner 
I was led to find that the higher classes of plants contain a mixture 
of two different kinds of chlorophyll (blue chlorophyll and yellow 
chlorophyll), by observing that the spectrum of the light of fluo- 
rescence of the solution showed two narrow red bands related to 
two absorption bands, and that on exposure to the sun one of these 
