1 885. J 
Spontaneously Reversible Spectral Rays. 
349 
IX. ON THE SPONTANEOUSLY REVERSIBLE 
SPECTRAL RAYS, 
AND ON THE 
ANALOGY BETWEEN THEIR LAWS 
OF DISTRIBUTION AND INTENSITY, WITH 
THOSE OF THE RAYS OF HYDROGEN. 
A. CORNU, in studying the spectroscopic reactions 
• of metals, has come upon some most interesting 
results, which he has recently laid before the 
French Academy of Sciences. 
The progress of spectroscopic research, and the wonderful 
extension of its applications, have led up to one of the most 
beautiful problems ever presented to the experimentalist and 
the generaliser. If we have given to us the chemical com- 
position of a vapour raised to incandescence under definite 
conditions, the question is to foresee the distribution of the 
spedtrum, of the radiations emitted and their relative 
intensity. 
Though the attention of observers has often been called to 
this problem, its solution is by no means very near at hand. 
M. Cornu, therefore, proposes, after having briefly sketched 
the points _ where previous investigations have broken 
down, to point out an order of phenomena which seem to 
open out a novel track. 
The starting point for all researches of this kind is the 
following observation When we examine the luminous 
spedtra of incandescent vapours we often distinguish groups 
of rays which repeat themselves with a periodicity more or 
less regular. These repetitions are still more striking if we 
extend the scope of our observations up to the extreme limit 
of the ultra violet spedtrum. 
The analogy between these periodic groups and the har- 
monics of sonorous bodies presents itself at once to the mind. 
The notion is so much the more captivating because the 
positions of these groups, if considered either on the scale 
of wave-lengths, or on that of the numbers of vibrations, 
seem to coincide with the successive terms of the series of 
whole numbers. It is no wonder, therefore, that so many 
spedtroscopists have yielded to the temptation and have 
sought to arrange the rays of one and the same spedtrum 
according to a harmonic series. 
