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. Especially 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, 1903-1910.3 



The sun's temperature. — If 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 : 



A,n„T=2930. 

 If /W=0.470 M 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 : 



76.8X10 



'"'^( uS'oQo ) 2 T4=L922 T=5830 ° 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. 



