i88o.] T/is Phenomena of Fluorescence . 631 
rays were direaed upon the liquid. An achromatic lens 
was then placed between the surface of the fluid and the 
slit : a coloured image of the slit upon the liquid surface 
was thus produced. With another refteaing prism the 
fluorescent light was directed into the slit of the collimator of 
a Brunner’s spectrometer. On looking through the instru- 
ment two coloured images appeared in the field of view ; 
one was the fluorescent light, and the other that reflected 
direCtly at the surface of the fluid. Nothing more then re- 
mained to be done than simply to measure the minimum 
deviation of these two images. Each of the experimental 
measurements showed invariably “ that the fluorescent 
substance has a lower refrangibility than the incident light.” 
Quite recently, however, Hagenbach has announced that 
after a prolonged investigation he has fully come to the 
conclusion that at present there exists no reason for rejecting 
the law of Stokes, and, moreover, that Lommel’s classifica- 
tion of fluorescent bodies is not based upon any essential 
difference in their behaviour. 
But the question as to the praaical utility of these disco- 
veries naturally arises. Science has a satisfaaory reply. 
When each of these fluorescent bodies are examined with 
the spearoscope they exhibit well-marked and charaaer- 
istic absorption-bands or dark spaces ; consequently we have 
here a most subtle means of deteaing their presence in a 
mixed solution or in a free state. Not only so, but Prof. 
Stokes has pointed out that if, on examining a solution in a 
pure spearum, “ we find the fluorescence taking a fresh 
start with a different colour, we may be almost certain that 
we have to do with a mixture of two different fluorescent 
substances, the presence of which is thus revealed without 
any chemical process.” 
Nearly two years ago, however, a most singular inquiry 
was brought to a satisfaaory issue by the aid of fluorescence. 
It was long a question whether or not the head waters of 
the Danube found their way through subterranean passages 
into the Aach. The problem was thus solved: 10 kilos, 
about 22 lbs.) of fluorescein were poured into the first-named 
river, and three days afterwards a splendid green colouration 
with’golden refkaions (i.e., its fluorescent effea) was quite 
distina in the waters of the Aach. It was estimated that 
in this way 200 millions of litres of water were coloured by 
the presence of this substance. Doubtless Science will 
apply these beautiful phenomena to other and even greater 
ends than those which have yet been attained. 
