on the Temperature of the Sun. 549 



degrees Centigrade, is 



£ = 9965°-4, 



if we only take into consideration the absorption produced by 

 the terrrestrial atmosphere. If we neglected this absorption 

 we should have a lower temperature. In short, in the obser- 

 vations made, the maximum was obtained on September 28th 

 at midday : this is represented by 210 scale-divisions, which 

 gives for y the value 



# = 5'6921x 210 = 1195-3. 



If we introduce this value into the formula, we obtain 



T= 8883-8, 

 giving 



£ = 8610-8. 



This result will be greatly modified if we take into 

 account the absorption exercised by the solar atmosphere. 

 According to Secchi, the solar atmosphere exercises a very 

 powerful absorption on the rays which proceed from the photo- 

 sphere : on account of this absorption only -£$q of the solar 

 radiation pass beyond the atmosphere of the sun, whilst -^fa 

 are absorbed by it. If we regard this value given by Secchi 

 as correct, we can calculate the thermal effect which the sun 

 would produce if it were without atmosphere. This effect 

 would be 



100 



y = 1838-5x^f =15320-8. 

 9 12 



The formula gives 

 and consequently 



T= 20653-7, 



£ = 20380-7. 



There are still two causes which can modify these results ; 

 but certainly their effect is slight, since their influences are 

 contrary and so compensate one another. One of these causes 

 is the value of the specific emissive power of the sun, which may 

 possibly be less than unity ; and in that case the true tempera- 

 ture of the sun would be higher. The other cause is the transpa- 

 rency of the different strata of the solar atmosphere : although 

 this is small, it is nevertheless certain that Ave receive the rays 

 from several superposed strata : and although their tempera- 

 ture is certainly lower than that of the photosphere situated 

 underneath, nevertheless to the radiation of the latter a portion 

 of the radiation of these strata joins itself; and consequently 

 in that case a lower temperature of the sun is sufficient to 

 produce the heating measured by our instruments. 



