-~ 
188 S. P. Langley—Determination of Wave-lengths. 
through all the details of which his aid has been more that 
of a coadjutor, than an assistant: 
. 
Allegheny Observatory, Allegheny, Pa., October, 1883. 
article by Mr. H. Becquerel in the Annales de Chimie for 
September, 1883. 
The wave-lengths assigned by M. Becquerel to the band at the 
limit of his researches at 1-460 to 1-480 appear to me too great, 
for this limit corresponds to the diffuse margin of the wide band 
the wave-length of whose coldest part is given at 1“:36 to 1:37 
on my chart, published in the Comptes Rendus of the previous 
a (Sept. 11, 1882) and on a larger scale in the American 
ournal of Science for March, 1883, and in the Annales de Chimie 
Norr.—Since the above was in type I have seen the interesting 
Hid 
lengths 1-80 to 1°-90, while M. Becquerel’s farthest band asI- 
have said is at 1-48 (according to him; but according to. my 
measures more nearly at 1-38). The present memoir will show 
what degree of reliance may be placed on these measurements. 
It is understood that a photographic map of the spectrum to 
1"-6, and therefore covering the ground of M. Becgnen’ 
but not extending as far as my (Q, will shortly be published from 
may it 
least, of the points in question. 
would call attention to the fact that M. Becquerel has stated 
that the farthest band known to him in Sept., 1883 (except from 
my own researches), had a wave-length of not over 14°50, accord- os 
ing to his own estimate. a 
