ON THE EFFECTIVE TEMPERATURE OF THE SUN. 
305 
used in or near the tropics, where atmospheric conditions can be trusted to remain 
more constant than in this country. Any error in the absolute value obtained might 
probably be considered constant, so that comparative values from year to year might 
be trusted to indicate any change. 
Note, added April 13th, 1894. 
It has been mentioned in the paper that Rosetti’s determination of the amount of 
the (terrestrial) atmospheric absorption has been used in the calculations of the 
effective solar temperature. It may be well, however, to give the result obtained by 
using other estimates of this quantity, which (after the law connecting radiation and 
temperature) is the most important factor in the final value. 
Taking Langley’s estimate for zenith absorption, 41 per cent., instead of Rosetti’s, 
29 per cent., the respective transmission coefficients being therefore 59 per cent, and 
71 per cent., the temperature would l)e multiplied by iy(71/59) approximately; 
i.e., instead of 6200°, we should obtain 
6200 X ^(71/59) = 6200 X 1-054 = 6535° 0. 
But a later, and still higher, estimation of the zenith absorption has been made. 
Angstrom (‘Wied. Ann.,’ 1890, vol. xxxix., p. 309) has shown that the effect of the 
carbonic acid gas in the atmosphere is much more important than had hitherto been 
supposed, and obtains 64 per cent., as against Rosetti’s 30 per cent, and Langley’s 
41 per cent. This gives 36 per cent, as the transmission coeffieient, and, taking this 
value, the temperature becomes"^ 
6200 X ^(71/36) = 6200 X -1^(2) approximately = 6200 X 1‘189 = 7370°. 
And, to make the case general, if any later investigation shows the zenith trans¬ 
mission coefficient to be X per cent., the effective temperature becomes 
6200 X v'(71/X). 
It may also be of interest to see what effect is produced if absorption in tlie atmos¬ 
phere of the sun itself is taken into account. First, considering the falling-off in 
radiation from the central to the peripheral parts of the sun’s disc, from Wilson and 
Rambaut’s paper “On the Absorption of Heat in the Sun’s Atmosphere” (‘Proc. 
R.I.A.,’ 1892, 3rd series, vol. 2, j)- 299), we may deduce that, if the absorption were 
* The ratio of tlie zenith-absorptions is practically equal to that of those with a greater thickness of 
atmosphere, at least clown to a zenith-distance of 50°. 
3 E 2 
