S. P. Langley—The Solar Atmosphere. 491 
then is a nearly neglected one, and the methods of investiga- 
tion of less apparent interest than those of the spectroscopist, 
the results are, if verified, of a peculiar interest, from their 
bearing upon the possible changes in the mean temperature 
and climatic conditions of our own planet. 
The portion of this atmosphere chiefly concerned in absorp- 
tion I have been led to believe from several considerations is 
extremely thin, and I am inclined to think it is nearly identi- 
cal with the “ reversing layer” at the base of the chromosphere 
observed by Secchi and Young, though the other chromospheric 
strata doubtless, have some share in the obscuration made, 
and, in a still less degree, the other solar envelo 
e methods used for heat measurements at Allegheny have 
been partially described,* and need not be enlarged upon here. 
Those for light are believed to be novel in their present appli- 
cation, and may be given briefly. 
e have seen that a photometric comparison of the center 
and edge of the sun is for some reason attended with risk of 
error which we should not anticipate, but which must be pce 
from the enormous discrepancies existing oe co ob- 
i , Liais’ is four 
hundred a cent, and Secchi’s three thousand per cent greater. 
stance, by the familiar methods of ordinary photometry, need 
hot be expected, as common experience shows, to commit er- 
rors which are in any way comparable to these. 
If such familiar means as the Rumford and Bunsen hoto- 
At first sight it a pears that these methods are not applica- 
ble to the divect pics preci for the ordinary use of either we 
supposes that the relative distances of the lights f c 
the Screen can be altered, while we cannot chan the relative 
, ces of the lights to be here compared, which are two por- 
tions of the sun; and, if we attempt to enfeeble the stronger 
lights by shades, or by diffusing it through a lens till it equals 
that of ‘the other seen direct, we lose the peculiar advantages 
of the methods which assume that the lights are viewed under 
the same conditions and measured by the relative distances 
. sends zed, im apparatus which to be 
- . in June, 1874, an tus which appears to t 
ae thee frcan’ these objections, and ‘chich is bere ribed in 
, * Comptes Rendus, May 22, 1875. 
