S. P. Langley—Selective Absorption of Solar Energy. 178 
_ blacked surface. It will be seen hereafter that the whole ex- 
_ periment has been repeated with a lampblacked bolometer 
_ without in any way affecting the results. The wave-lengths 
_ are derived from the measured angles by 1 
_ the use of the formula ns = sine 7 + sine pms 
r, where n is the order of the spectrum, s s 
a the space between the lines of the grat- 
ing, 4 = the wave-length of the radiation, 
i the angle of incidence (in the present 
q ence 0°), and r the angle of diffrac- 
am the early observations it appear 
m the examination of the di fraction 
‘spectrum up to A= 1"-0 that the energy 
in the invisible part as far as this, was 
Nothing 
_toan regen) little _ r than this; and 
One of those most conversant with the 
subject has lhe this wave-length (i. e. 
10) as marking the limit of everything 
_ known to exist.* 
It seemed at first then improbable that 
the heat below the red should materially 
_ exceed or even equal that above it; for 
this would demand (since the heat shown 
by the last ordinate at 4= 10 is very 
small) an extension of the curve of ne 
as obtained from the grating, to a 
tance enormously beyond the favhelt 
a 
* Draper, “On the Phosphorograph of a Solar Spectrum, and on the Lines in 
the Infra-red Padi Sesinsiges mee © of the American Academy, vol. xvi, p. 233, 
0. “Do we not encounter the objection that ve wave-length 
10°750m-10 ee. limit of Captain Muiey’ 8 rtd is altogether beyond the theoretical 
limit of ne prismatic m?” Previous measurements of nor had, it will 
be remembered, been made by comparing 3 its total amounts, in the visible ‘and in- 
visible Siebate spectrum, which gives us no knowledge as to wave-lengths in 
any case, and w: wave lengths = >= dark-heat — n, had been estimated, by ha 
ahdous extra-polati radictory formule —formule which profess a the- 
oretical basis, but prin t geo other. Thus Miller finds by Radtenbacher’s 
formula a wave-length of nearly 5/-0 for the extreme solar heat rays, Draper (aS 
st seen) a wave-length of spe oe 0 for nd a rays, etc. All these 
servations in the visible 
Ee 
8 
i) 
i 
zg 
B 
ee 
Bs 
" 
4: 
rays, whose real place we give later 
