1894.] 



The Thermal Radiation from Sun Spots. 



249 



The mean of the two photospheric readings he used as a divisor for 

 the umbral reading. He then says, " The decrement of heat as we 

 approach the limb is, though not exactly, yet so very nearly, in the 

 same ratio for photosphere and spots, that no correction is needed on 

 this account for the present observations." 



If Langley failed, through want of instrumental means, to notice 

 the difference between the absorption in a spot and the photosphere 

 near the limb, his method would make his umbral readings too high. 

 The mean of twenty observations here equals 0'356, against Langley's 

 0*54. This is a serious difference, and, I think, can only be accounted 

 , for either by the use of superior instrumental means, or by a possible 

 variation in the radiation of spots in different years of the sun spot 

 cycle. 



It is difficult to see how too low a value for nmbral radiation could 

 be got, whereas too high a one might be found by want of definition 

 and trembling in the image, so that some of the penumbral radiation 

 would reach the thermo-couple. 



