490 S. P. Langley—The Solar Atmosphere. 
case founded on an experimental comparison of the light or 
heat of the sun observed at the center with that observed near 
the edge. 
This direct comparison of the two lights involves, of course, 
no hypothesis, but is simply a photometric measurement, and 
apparently aneasy one. Let usexamine the result of this com- 
parison at different hands. Arago,* using the Rochon prism 
and analyzer, announced, as the conclusion of a prolonged 
research, that the light at the center of the solar disc must be 
diminished by ,', part, or 2°4 per cent, to equal that of the edge. 
Liais in a memoirt+ which is a model of conscientious care, an- 
nounces, from comparisons carried on by the extinction of one 
light by a stronger, that the central light should be diminished 
by about 10 per cent, to equal that of the edge. His result is 
then four times as great as Arago’s. Secchi with a wheel pho- 
tometer, finds, for a nearly corresponding point, 78 per cent as 
the amount by which the central light should be diminished. 
This result is then seven times that of Liais and ¢hirty times 
that of Arago. : 
Secchi has made observations on radiant heat to be found in 
Le Soleil,t the most important results of which are that the heat 
diminishes from the center to the edge; that for a given point 
there isa satisfactory agreement between the absorption of light 
and heat ; and that the equatorial regions are hotter than the 
polar. The present writer has made somewhat extended re- 
searches in the same direction which are chiefly unpublished. 
Vogel has made interesting researches in the relative actinic 
absorption. 
None of those who have attempted to compute the amount 
of the solar absorption appear to Live drawn conclusions from 
their results as to its effect on terrestrial temperatures, and yet 
if the absorption be anything like what has been found by La 
Place, and by Secchi using ie Place’s formule, the subject de- 
serves attention. 
this planet largely depend upon it. It will further appear 
probable that Lang slight sates in the depth and absorptive — 
power of this atmosphere, fluctuations in terrestrial tem ne 
will ensue, very great in comparison with any actually observed = 
_ within historic periods, and it will be shown that this atmos — 
phere is not in a strictly stable condition. Though the subject — | 
: * Oeuvres ro ye Tome i, p. 235 | 
+ Mémoires de Académie de Uherbourg. : 
_ Le Soleil, 2me edition, Paris. 1875. 
