/ 



294 Mr. S. P. Langley on the Amount 



This neglect to make what seems so pertinent an applica- 

 tion of Melloni's observation, even after it had been explained 

 and extended (by Biot), will seem more explicable when it is 

 remembered that no direct means of measuring the absorption 

 in even approximately homogeneous rays till very recently 

 existed, and that a departure from the old formula, which 

 ignores the difficulties, involves their recognition, and the 

 devisal of new processes to meet them. Even if we, by the 

 employment of such new processes, succeed in measuring the 

 absorption in approximately homogeneous rays, the approxi- 

 mation is chiefly to homogeneity in wave-length, and not to 

 uniformity of physical properties in consecutive wave-lengths, 

 so that we are unable to represent the absorption as any con- 

 tinuous function of the latter. In other words, we may 

 measure on separate narrow portions (AX b A\ 2 , &c.) of the 

 spectrum, and determine for each its apparent coefficient of 

 transmission (p 1} p 2 , &c), which is in each case some func- 

 tion of the wave-length ; but we are not at liberty to write 

 that the original energy of the heavenly body 



■r 



— e 



(<j)\) d\, 



since our (f>\ is really discontinuous, a remark the import of 

 which will become more apparent in the sequel. For the 

 present at least we are at liberty only to divide the spectrum 

 into a finite number of equal parts, and to mechanically sum 

 them. 



I have already stated elseAvhere * that in neglecting the 

 fact that the absorption is really selective we not only commit 

 an error, but an error that always lies in one way, so that any 

 determination of the absorption we make by the ordinary and 

 erroneous formula never errs by being too great, but is, so far 



little treatise ' Actinometrie ' by M. Radau. The use of two coefficients 

 is proposed in this, as it has been before, but does not seem to have been 

 followed by others, who, like M. Violle, have subsequently (Annates de 

 Chimie et de Physique, 1879) employed but a single coefficient of trans- 

 mission. Still more lately, however, the importance of the consideration 

 on which the writer here insists has been remarked on by Messrs. Lecher 

 and Pernter, and perhaps by others. The employment of the method of 

 Forbes, especially as modified and extended by Crova, appears to be the 

 best means at the command of the observer with the actinometer or pho- 

 tometer. This method, however, is unfortunately very limited in its 

 practical application, owing to the insufficiency of data thus obtainable, 

 and it still gives a necessarily too small result, though a larger one than 

 Pouillet's. 



* Comptes Rendus de VAcad. des Sci. xcii. p. 701 (March 21, 1881). 



