OP SUNLIGHT THROUGH THE EARTH’S ATMOSPHERE. 
263 
Z.D. 
Sec Z.D. 
Fortes’ Value. 
Bouguer’s Value. 
0 
1-0000 
1-0000 
1-0000 
10 
1-0154 
1-0164 
1-0153 
20 
1-0642 
1-0651 
1-0642 
30 
1-1547 
1T556 
1T547 
40 
1-3054 
1-3062 
1-3050 
50 
1-5557 
1-5550 
1-5561 
60 
2-0000 
1-9954 
1-9903 
70 
2-9238 
2-9023 
2-8998 
75 
3-8637 
3-8087 
3-8046 
80 
5-7588 
5-5711 
5-5600 
82-30 
7-6613 
7-2343 
— 
85 
11-4737 
10-2165 
10-2002 
86 
14-3356 
12-1512 
12-1401 
88 
28-6537 
18-8825 
19-0307 
90 
Infinite 
35-5034 
35-4955 
Astronomers have made various estimations of the value of what they term 
absorption, and perhaps no astronomical problem has received more attention than 
this one. The determination of the coefficient of absorption is a necessary preliminary 
for ascertaining star magnitudes, and thus has an importance peculiarly its own. 
Professor Langley, to whose work I shall presently have to refer, made two estima¬ 
tions, one on Etna at 4,000 feet elevation, and another at Mount Whitney, at a still 
higher altitude. At both of these localities he found a value for the coefficient of 
transmission to be '88, though at the latter station he disclaims any great accuracy 
as likely, which is indeed the case, considering the method he caused to be employed. 
The other values obtained were as follows :— 
Pritchard at Cairo and at Oxford, ‘843 and 791 respectively; Bouguer ‘812; 
Seidel 794; and Muller - 825 : or a mean of ‘804 at low-level stations. 
Professor Langley, assuming from his observations with the bolometer, regards 
these results as being liable to error. He says, “ For be it observed in general 
terms that, since the rays with large coefficients are represented by diminishing 
geometric progressions, whose common ratio is near unity, these rays will persist, 
whilst others with small coefficients are early extinguished. 
But what we desire now further to point out is that, according as the difference 
of these coefficients of transmission for the different portions of the light of the same 
star is greater, so will the error of the result in treating them as equal be larger : a 
consequence so obvious that it is only necessary to make the statement in order to 
have its truth recognised. 
“ Since it has now been demonstrated that the formula ordinarily employed leads to 
too small results, it might properly be left to those who still employ it to show that 
their error is negligible ; but this has never been done. There is possibly an impres¬ 
sion that if there were any considerable error its results would become apparent in 
such numerous observations as have been made all over the world in stellar photo- 
