1894.] On the Effective Temperature of the Sun. 251 



tion from it may be increased so far as to equal that arriving at the 

 radio-micrometer from the sun. 



The following points are considered, after descriptions of the 

 method and apparatus have been given : 



1. The law connecting radiation and temperature. 



This is probably the most important factor in the value of 

 the final result. Numerous investigations on the point have 

 been made, which are referred to in the paper. 



After a careful series of experiments we have come to the 

 conclusion that (at least for bright platinum) Stefan's " law 

 of the 4th power " holds,* i.e., that for high temperatures (say 

 over 600 or 700 C.) if R = the radiation from a source whose 

 absolute temperature is T, then 



RocT 4 . 



2. The emissive power of platinum at high temperatures compared 



with that of lamp-black. 



On this point the value obtained by Rossetti was used, 

 some considerations being given in support of his figures, 



3. The amount of the atmospheric absorption. 



This is fully discussed, and again the value obtained by 

 Rossetti is used. 



Langley's theoretical value for percentage absorption of 

 radiation from a body in the zenith, viz., 41 per cent., is shown 

 to be possibly too great ; Rossetti obtained 29 per cent., which 

 appears to be the value best supported by experiment. 



The climate of Ireland entirely prevents a systematic series 

 of investigations on this particular point. 



Several subsidiary questions are also discussed, and, finally, the 

 results of about sixty-nine observations are given, which lead to a 

 final mean result for the effective solar temperature of 6200 C. 



It is pointed out, in conclusion, that the method would probably 

 give excellent results if adopted in some country in or near the 

 tropics, where atmospheric conditions can be trusted to remain more 

 constant for some weeks, or even days, together, and where a series 

 of observations taken at the same part of the year throughout the 

 period of a sun-spot cycle might be hoped to settle the question of 

 how (or if) the solar temperature varies during this time, as any 

 error in the absolute value obtained may probably be considered con- 

 stant, so that comparative values from year to year might be trusted 

 to indicate any change. 



* Stefan, however, stated the law as applying to the " pure " radiation of a 

 surface of perfect emissive power. 



