248 Mr. W. E. Wilson. [Mar. 15, 



depth in the solar atmosphere through which the radiation would 

 have to pass. On the other hand, if the spot was floating above the 

 absorbing atmosphere the radiation from it would remain constant in 

 any position on the solar disc. 



The following is the value of the heat radiation from the photo- 

 sphere taken along a radius of the sun, where = centre and 100 

 the limb. The radiation R equals 100 at the centre.* 



D. R. 



100-0 



10 99-8 



20 99-5 



25 99-3 



30 98-9 



40 97-2 



50 95-3 



60 92-2 



70 87-8 



75 85-3 



80 82-5 



90 72-0 



95 61-8 



98 51-5 



100 42-9 



It will be seen by the following observations of spots, taken from 

 August 5 to November 9, that there is distinct evidence that the 

 radiation from the spot does not fall off as rapidly when near the 

 limb as the neighbouring photosphere ; in fact, the ratio w/C remains 

 nearly constant, whereas the ratio /N gets nearer unity as the spot 

 approaches the limb. The spot observed on the 22nd of October is a 

 good example, as the same spot was observed again on the 26th, 29th, 

 and on the 30th, when it had reached within a distance, D, of 95 

 from the centre. It will be seen that on these four dates the ratio 

 w/C was respectively 0'338, 0'360, 0'313, 0'356, whereas the ratio u/N 

 was 0-349, 0'410, 0706, 0-783. 



Langley,t in 1874 and 1875, measured the radiation from sun 

 spots. He used a thermo-pile and galvanometer, and obtained as the 

 mean of his results a ratio of 0'54 + 0'05. 



His method was first to take a reading in the neighbourhood of the 

 spot, but between it and the centre of the disc. He then took a 

 reading in the umbra, and, finally, a third reading in the neighbour- 

 hood between the spot and the edge of the sun. 



* " The Absorption of Heat in the Solar Atmosphere," by W. E. Wilson and 

 A. A. Rambaut, ' Proceedings of the Royal Irish Academy,' 3rd series, vol. 2, No. 2. 

 t ' Monthly Notices,' vol. 37, No 1. 





