2 1 MM. Lecher and Pernter on the Absorption 



here to do with wave-lengths which are almost all greater 

 than those which have been examined in the laboratory. 



Hence all conclusions and remarks in this direction are 

 only admissible on the assumption that the absorption-ratios 

 found hold good for the neighbouring less-refrangible region 

 of the spectrum ; and this, however probable it may appear, 

 has never been proved. 



The opposite generalization, to the case of the absorption of 

 the sun's radiation is just as little justifiable ; only here there is 

 somewhat more reason for the proceeding, since some of the 

 wave-lengths in the sun's radiation coincide with those exa- 

 mined by Tyndall. That portion of the total solar radiation 

 must of course first be found which is caused by waves of 

 shorter length than those of Tyndall's source of heat (270°). 

 It is at present only possible to make a very rough estimation 

 of this ratio, which may perhaps be reached as follows : — The 

 incandescence of a body begins, according to Draper*, at a 

 temperature of 525° C. In order to find the ratio between 

 the radiation at 270° C. and at 525° C. we will employ a law 

 which Stefan f has stated as the result of all the experiments 

 made in this direction. 



The quotient is found to be 



(273 + 270) 4 -(273) 4 : (273 + 525) 4 -(273) 4 ; 



that is, as 1 : 4. The radiation of the body increases, there- 

 fore, in the ratio of 1 to 4 when its temperature is increased 

 from 270° to 525°; but the greater radiation is partly due to 

 increase in the intensity of the old wave-lengths. We have 

 therefore the discrepancy that at least the fourth part of the 

 radiation of a blackened surface at 525° C. is due to wave- 

 lengths which were already present at a temperature of 

 270° C. Now Wm. W. Jacques J has shown that from the 

 temperature of low redness up to the most intense white heat 

 the relation of the energy of radiation is nearly the same. 

 Consequently, at least the fourth part of the dark radiation 

 must be produced by rays such as Tyndall investigated. If 

 now we assume, in accordance with the experiments of 

 Miiller§, that the dark portion of the sum's radiation is only 

 twice as great as the luminous portion, we shall have at least 

 f X J of the intensity of the total solar radiation due to such 

 wave-lengths as Tyndall employed. (If the emission-coefficient 

 of the sun were not equal to unity, the above fraction 



* Phil. Mag. (4) xxx. 



t Sitztwiffsberickte, lxxix. Abth. 2. 



J Proceedings of the American Academy, new series, vol. vi. 1879. 



§ Pogg. Arm, CV. 



