i88 5 .] 
35i 
Spontaneously Reversible Spectral Rays. 
With these ideas M. Cornu has for a long time pursued 
the study of the visible and of the ultra-violet spedtra. He 
has succeeded in detecting a class of spedtral rays possessing 
characters so distindt that they cannot be confounded with 
any others. These are the spontaneously reversible rays which 
M. Cornu described in the “ Comptes Rendus ” for 1871 
(vol. lxxiii., p. 332), and the existence of which has been 
demonstrated in the spedtra of most of the metallic vapours. 
The common disposition of their groupings, as will appear 
below, seems to define one of those experimental forms of 
fundtion answering to the conditions above announced. The 
following are the fadts : — 
The spontaneously reversible rays are well known to 
physicists, and were observed for the first time by M. Fizeau, 
in the combustion of sodium. An incandescent vapour of 
small density and at a low temperature emits a certain 
radiation which corresponds in spedtral observation to a 
certain brilliant but very narrow ray. If we increase pro- 
gressively the temperature and the density of the vapour, the 
ray grows both in intensity and in width and becomes a true 
luminous band. Soon we see appearing upon this band a 
dark line in the place of the primitive fine ray, and finally 
the luminous band broadens in a manner somewhat indefi- 
nite, still presenting also the dark line, which also expands 
in a manner about proportional. Upon the luminous ground 
produced by the expansion of the band, the brilliant, non- 
reversible rays vanish completely. 
These phenomena, observable in the visible spedtrum with 
the majority of the volatile metals, and with the indudtion- 
spark, reach enormous proportions with the eledtric arc, 
especially in the ultra-violet region. There may be 
mentioned in particular the ray A = 228‘85 of cadmium. 
This ray, extremely weak with the induction spark, expands 
in the arc so as to invade almost the entire photographic 
spedtrum. As for the dark reversal band which occupies its 
centre, it comes to occupy more than half the interval 
between the rays 23 and 24 (Mascart’s notation), which dis- 
appear almost entirely. We may further mention the ray 
X = 209 of zinc ; two of the components of the quadruple ray 
of magnesium A = 280, as also certain fine groups in the 
spedtra of aluminium and thallium. 
Various observers, above all Messrs. Liveing and Dewar, 
have observed a great number of curious reversals in the 
most various circumstances. The most interesting, perhaps, 
are those of the hydrogen rays in the visible spedtrum, and 
in the ultra-violet that of the ray A = 285 of magnesium, 
