66 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
to 1910, and have been made with many different optical systems. There is 
great difficulty in getting an accurate estimate of the relative losses suffered 
by rays of different wave lengths in traversing the spectroscope. TEspecially is 
this the case for the violet and ultra-violet rays, where these losses are greatest. 
The summary has shown that further determinations are needed to fix the dis- 
tribution in the extreme ultra violet, and observations for this purpose were 
made in June, 1911, on Mount Wilson, but are not yet reduced. I give below 
the summary, excluding the work of 1911. 
Intensities in normal solar spectrum, outside the atmosphere. 
[Observed at Washington, Mount Wilson, Mount Whitney, 1903-1910.] 
7 u u I ut ut Ke 
Wiavelengther seen c- + sree 0.30 0.35 0.40 0. 45 0.47 0.50 0.60 
Beiutonsity sa ete st. enone 440 2,700 4,345 6, 047 6,253 6, 064 5,047 
Probable error (percentage)... .- 50 (?) 7.3 15 1.4 1.8 1.9 Qik: 
Wavelength? 22. cos. san 55-2 a8 0. 80 1.0 1.3 1.6 2.0 2.5 3.0 
Intensity 22 -4--seewese see oe 2,672 1, 664 897 526 245 43 12 
Probable error (percentage)..-.- 12 0.7 0.7 1.4 2.4 4.8 45(?) 
The sun’s temperature—If{f we employ the so-called “‘ Wien displacement 
formula,” which connects the absolute temperature of a perfect radiation with 
the wave length of its maximum radiation, we may proceed as follows, to esti- 
mate the solar temperature, on the assumption that the sun is a perfect 
radiator : 
AmaxT=2930. 
Tf Amx=0.470 pc then T=6230° abs. C. 
Another radiation formula is that of Stefan, which connects the total quan- 
tity of radiation of a perfect radiator per square centimeter per minute with 
the absolute temperature. Employing this formula, still assuming the sun to 
be a perfect radiator, its mean distance 149,560,000 kilometers, its mean diame- 
ter 696,000 kilometers, and the mean value of the solar constant of radiation 
1.922 calories per square centimeter per minute, we proceed as follows: 
696,00 . 
76.8X10-?X (sa sy 00) T1922) L—58302 abs: C: 
A third means of estimating the sun’s probable temperature comes from com- 
parisons of the distribution of the energy in its spectrum with that in the 
spectrum of the perfect radiator, as computed according to the Wien-Planck 
formula of spectrum energy distribution. The sun’s energy curve and that 
of the perfect radiator at two temperatures are given in the accompanying 
illustration (fig. 2). It appears at once from this comparison that the sun’s 
radiation differs greatly from that of the perfect radiator at any temperature. 
The solar radiation is greater in the infra-red spectrum, and much less in the 
ultra-violet spectrum, than that of perfect radiators giving approximately the 
same relative spectral distribution as the sun for visible rays. Taking every- 
thing in consideration, the solar energy spectrum seems most comparable with 
that of a perfect radiator between 6,000° and 7,000° in absolute temperature. 
The causes of the discrepancies we have noted may be several. First, there 
is the influence of the selective absorption of rays in the Fraunhofer lines, 
