( 276 ) 



II. TllK VAKIA151L1TY OF THE SOLAH KADIATION. 



It is well known tluil the composition as well as the intensity of 

 the Sun's radiation is inconstant. As to the variation of the total 

 inlensity, this coukl iiot l)e ascertained by actinonielrical nieasiire- 

 ment, owing to tlie capricious disturbances caused by the clouds; 

 it has therefore l)een determined in an indirect way, from the values 

 of the mean temperature all over the Earth. But the variability in 

 the composition of the light has been revealed by a careful study of 

 the Fkalnhofeu lines which has siiown tliat several lines are atone 

 time more enhanced than at others (Jewell^) Hale"), Langley')). 

 In the spectrum of sun-spots also, in which several lines are com- 

 l)aratively very wide, N. Lockyer '') noticed that the mean ty[)e of 

 the spot-spectiMini undergoes a peritxHcal uiotHlication, the pei'iod of 

 which coincides with that of sun-spot frecpiency. 



As yet we have no certain indications that this [)eriodicity also 

 exists in the varying aspects of the Fraunhoeer lines of the average 

 photosphere specti-iini. The abnormal spcclruui photographed by 

 Hale ^) in 1894 (i.e. at a sun-spot maximum) presented, as has been 

 shown elsewhere "), this peculiarity that the lines, which in the chromo- 

 sphere spectrum are generally strongly marked (principally belonging 

 to Fe, H, Ca, Sr, AI, Ti), were very faiut, whilst the strong lines 

 (belonging to Zr, Mn, Y, V and some of unknown origin) did 

 not correspond to any of the chromospheric lines. 



The periodical \ ariability observed by Lockyer in the spot s|)ectrum 

 consisted herein, that when at spot maxiuium the most eidianced 

 lines were selected, they proved for the greater part to be "unknown 

 lines" i.e. lines which in the noi-mal solar spectrum are extremely 

 weak, and that the strong lines of Fe, Ni, Ti, which during minima 

 of spot periods often appear very wide, were then scarcely x'isible. 



1) Jewell, 4stroph. Journ. 3, 89-113, 18%; 11, !234-240, 1900. 



2) Hale, Astroph. Journ. 3, 15G-161, 189G; 16, 220-233, 1902. 



3) Langley, Annals of the Astroph. Observatory of the Smiths. InsliL, Vol. I, 

 1900. On p. 208, 209 and 216 mention is made of irregular changes in llie heat 

 spectrum (especially in *, ■*" and n), which do not seem to be occasioned by 

 absorption in our atmosphere and are therefore the etl'ect of cosmic influences. 

 Langley's excellent method of investigation may prove of the utmost value in the 

 study of the variability of the Sun's radiation, as it gives directly comparable 

 values for the energy of the various kinds of rays in this important part of the 

 spectrum. 



i) Lockyer, Proc. Roy. Soc. 40, p. 347; 42, p. 37; 46, p. 385; 57, p. 199; 

 67, p. 409, (1886-1900). 



5) Hale, Astroph. Journ. 16, 220—233. 



6) W. H. Julius, Proc. Roy. Acad. Amst., IV, 589—602. 



