1 885.] Spontaneously Reversible Spectral Rays. 399 
expedt to find side by side. These are aluminium and 
thallium, whose atomic weights figure about the extremities 
of the list of elementary bodies. 
Analogy with the Distribution of the Rays of Hydrogen. 
M. Cornu does not stop to indicate the fruitless attempts 
at numerical calculations which he undertook with the view 
of representing each of these series by the substitution of 
the series of whole numbers in a simple function. He had 
abandoned these researches when the beautiful discovery of 
Dr. Huggins on the spedtra of the white stars brought back 
his attention to the subject, and opened up to him a new 
horizon. These spedtra present a common series of dark 
rays, — that is to say, reversed rays, — fulfilling precisely the 
conditions of distance and intensity which characterise the 
spontaneously reversible rays in the metallic spedtra. This 
series prolongs the series of the well-known rays of hydro- 
gen, C, F, G',h; we might then foresee that the entire 
series would participate in it. This has been since con- 
firmed by the researches of Vogel, though still leaving some 
uncertainty. The interest of this identification was such 
that M. Cornu undertook to verify it himself, which he has 
only just effedted. The experiment is not free from diffi- 
culty, but by taking more and more minute proportions to 
remove the impurities of the rarefied hydrogen gas he has 
seen the foreign rays successively disappear, and he has 
finally succeeded in obtaining photographic proofs offering 
the series of astral rays in all their purity. 
The following proposition summarises the result of these 
researches : — 
“ In the metallic spedtra certain series of sponta- 
neously reversible rays present the same laws of dis- 
tribution and intensity as those of hydrogen.” 
