( 291 ) 



Avhilst, especially in tlio lompci-alo /onos, fiio wiiil(M' miiiimiiiu will 

 be lower than that of the siiniiiiei'. 



Let lis here once nioi'e call to iiiiiid (lie optical sij^iiilicaiice of the 

 surfaces of discontiiniity. As a I'tile they impart a fjreatei- divergence to 

 the beams of liuiil which travel ch)sely aioiifi,' them; consetpieiilly at 

 theii- interseclion with I he surface of tlie Earth tiiey (h'termine zones 

 wiiere the iibiminalion will be weakened, whilst in tlie intermecHatc 

 regions it will be strengthened. This does not apply in the same 

 measure to eveiy kind of light in the sj)ech'nni, hul es|»e('ially to the 

 waves which undergo anomalous dispersion. 



All terrestrial phenomena which are governed l)y the conditions 

 of illumination will therefore, to a greater or lesser extent, be sub- 

 ordinate to the above-mentioned periodical variations. 



There is probably still another reason for the greater \arial>ilit\ 

 of the eifects of radiation in Sj)ring and autunni than in summer 

 and winter. It is namely not improbable that in regions 6° oi- 7° 

 distant tVom Ihe Sun's e(|uator greater differences of density Avill be 

 found along the surfaces of discontijinity, than in the equatorial zones. 



B. The jx'fiodiciti/ of the jl iirtiialions of ilhimliKitlon inli.lfh. 

 conicidi's icitli the periodicitii of soldf jilicnonieiKi. 



\\\ the coui'se of a certain number of years Ihe Earth describes 

 through the system of the surfaces of discontinuity a somewhat 

 com[)licated ])ath, which we have repi'esented by the track of P on 

 the sphere B. The Earth therefore continually conies under the 

 influence of another portion of the system; and the phenomena 

 a[)pearing on the Sun inform us whether, in a certain space of time, 

 the light on its way to the Earth passes more or less often closely 

 along surfaces of discontinuity. For this circumstance is intimatelv 

 connected with the fre(iuency of prominences and sun-s]>ols and wilh 

 the as|)ect of many of the FRArNUOFKR lines (especially in Ihe spol 

 spectrum). A so-called "maxinnnn of solar acli\ity" means, that the 

 Earth during that period has been many times intersected by the 

 prolongations of sharply delined surfaces of discontinuity, and all 

 the terrestrial phejioniena resiilling froui Ihe \ arialious of iljuminalioii 

 will then also be al a maxinnim. 



As to the nature of Ihe counecliou helwcen sun-spols and promi- 

 nences on I he one hand and Ihe valiu's of meleorological and mag- 

 netic vai'ialions on Ihe oilier, il has heen nnixcrsally concede<| thai 

 no oilier deliuiliou was possible hnl liiis: "lliat lliey were Ihe effects 

 of one and Ihe same common cause." 



We believe to have Ibinid this commoji cause in the var ving 



