628 
The Phenomena of Fluorescence . fOdtober, 
in castor oil : the most marked among these is camwood ; 
in this medium it shows a strong apple-green fluorescence. 
When paper is written upon with a tincture of stramonium 
seeds the characters are quite invisible by ordinary light, 
but appear self-luminous when exposed to highly refrangible 
rays, such as those given out by burning sulphur in oxygen. 
Fluoraniline (obtained by acting upon aniline with chloride 
of mercury) is said to be one of the most powerful fluorescent 
bodies at present known. 
The tindtures of several vegetable substances have a 
marked effect on light in this way : that of stramonium 
gives a pale but lively green fluorescence ; guaiacum, a beau- 
tiful violet colour ; turmeric, a greenish tint. Two artificial 
colouring matters, fluorescein and eosin, exhibit this pro- 
perty in a truly remarkable manner; but the effedt is most 
brilliant in the green and blue portions of the spedtrum 
(i.e., the rainbow band of colour obtained by decomposing 
light by a prism). 
A decodtion of madder in a solution of alum shows a high 
degree of sensibility ; it displays a copious yellow light. A 
weak solution of archil, which is red or purplish by trans- 
mitted light, gives a sombre green fluorescence. 
But Stokes found that the eledtric light (from carbon- 
points) gave a spedtrum, by the aid of a train of quartz 
prisms, which was more than six times the length of the 
ordinary spedtrum. This light is in fadt very rich in what 
are called the highly refrangible rays, and, moreover, its 
effedt on fluorescent bodies was strikingly beautiful, and 
especially in the case of uranium glass. 
As to the explanation of these phenomena, physicists are 
still somewhat at variance. But the experimental researches 
and elaborate reasoning of Prof. Stokes, who is at present 
the chief investigator in this department of inquiry, led him 
to the conclusion that beyond the visible spedtrum there are 
certain rays which ordinarily are not sensible to the eye, 
but when these highly refrangible or much-drawn-apart rays 
are allowed to fall upon certain substances the molecules of 
these substances are made to vibrate in slower periods than 
the exciting rays, these molecular vibrations readt on the 
ether, and consequently the rays on emerging (i.e., on re- 
flection) are hindered in their course and lowered in their 
refrangibility, that on being thus reflected they are brought 
within the range of vision : that is to say, if a ray of white 
light be made to traverse a prism, all those rays which go 
to make up the beam of white light, instead of passing 
through and being bent at the ordinary angle of simple 
reflection, are spread out like a fan ; the degree of divergence 
