’ 
189 SP. Langley— Determination of Wave-lengths 
lengths, as the deviation diminishes, and that, as far as we can 
-see at present, there 1s scarcely any limit to. the wave-length 
our prism can transmit except that fixed by rts absorptive effect. 
The approximate limit of the solar spectrum of the Hilger 
prism is at n=1°5485, which, according to Briot’s formula, cor- 
responds nearly to 8-4, but which according to our bolometric 
observations corresponds to an actual wave-length of 2":8. For 
this same point, as will be seen by Table IV, the values by 
Cauchy’s formula are impossible, and those by Redtenbacher's 
formula are imaginary. 
These last values rest, it will be remembered, on extra- 
polations founded on measures in the visible spectrum. 
Wave-lengths of Cold Lines in Infra-red Prismatic Spectrum. 3 
The following values (in table V) from Mouton, Abney 
and Draper are the ones I know previous to my own meas» 
ures where the wave-lengths of cold lines are given with 
most accuracy. Of these it is just to distinguish those by 
Abney as possessing a degree of exactness before unknown. 
There are some doubts about the band at 1-35 having really 
ose 
vaiues ’ 
first published in the Comptes Rendus, of the Institute of 
France, for September 11, 1882, in the form of charts, which 
were drawn from them. ese charts were so much reduced by 
the first engraver that though these values are still determinable 
from them, it may be convenient to repeat them here in thelr 
original tabular form, with the addition of the probable errors. 
Taste V.— Observed values of cold bands in infra-red by differ 
ent investigators. 
M. Movron.* W. DE W. ABNEY.t ~ J. W. Draper.t S. P. LANGLEY.$ 
0°824 0°815 0-835 0°815 +0°003 
0°850 0°854 0°85 40°003 
0°893 0-930 0:89 +0°004 
0°905 0-91 +0°004 
0°941 0-935 0-980 0-94 40°004 
0-985 0°975 0-983 113 4+0°007 
1-230 17240 1:27 +O" 
: . 0°00 
1-480 possibly Abney’s “yp” f } rae + 0-008 
1°54 40°009 X 
158 +0°009 ¥ 
181 +0°010\9 
1°87 +0°010 
1:98 +0°010 
2°03 +0°010 a 
* M. Mouton, Comptes Rendus, tome Ixxxix, p. 298; tome lxxxviii, P- 1190. 
W. de W. Abney, Phil. Trans. 1880, p. 653. 
ty. W. Draper, . Am. Acad. 1881, p. 223. 
§ S. P. Langley, Comp. Rend., Sept. 11, 1882; Am, Jour. Sci., March, 1 
~ 
383, ete 
