The Effective Temperature of the Sun. 317 



We need not inquire what is the absorptive power of the thermo- 

 j unction, provided that we are justified in assuming that lamp-blacked 

 surfaces absorb the radiation from the hot tube as freely as that from 

 the sun, or that the constants in these expressions (1) and (2) for the 

 radiation may be taken to be equal. 



On balancing, these expressions must be equal, and therefore 



(100) 2 ' 



(T \ 4 

 =^ ) may be neglected, hence we have finally 



04 iw 



ird 2 sin 2 p p 



V 10000 YljLl 

 v 7TC? 2 v ^sin 2 p 



0-13806 4 /7[ 

 J~s^~o ' v PQ' 



or 



pq sin- p 



The mean value of -^ - is [1-30413]. 

 v sin/a 



Therefore 0=1- 3041 3 . ? / JL . 



After a series of observations had been made, the furnace and tube 

 were raised so that the radiation of the latter then passed into the 

 aperture (A), on which the sunlight had previously fallen, while the 

 beam of sunlight was now directed so as to be upon (B), and in this 

 position a second series of observations was taken. The geometrical 

 mean of the result of the two groups gives the Effective Temperature 

 of the Sun, the effect of any difference in the sensitiveness of the 

 thermo-j unctions disappearing in the geometrical mean. 



Observations were made in the manner described above on August 

 19th and September 30th, 1901, and reduced by means of equation (4), 

 as exhibited in the following tables. In these the successive columns 

 contain (1) the local mean time, (2) the value of /? as read on the 

 micrometer head, (3) the absolute temperature of the tube in the 

 furnace, (4) the sun's altitude, (5) the percentage of the sun's radia- 

 tion transmitted through the earth's atmosphere, (6) the angle of 

 incidence on the heliostat mirror, (7) the percentage reflected from the 

 surface of the mirror, (8) the corresponding value of deduced from 

 equation (4). Of these (5) and (6) and (7) were determined as in 

 Wilson and Gray's memoir referred to above. 



z 2 



