( 283 ^ 



It seems expedioiil liei-e lo ouIcm- soniewlial fiii-lliei' iiilo i1i(M|iics(I()ii 

 as to how the pcM-iodical cliaii^e of position of our point of view 

 may cause tlie iiuml)er of spots seen in the course of a \qciv <i;ra- 

 dually to decrease foi' a pei'iod of (5 oi- 7 ycwi-s, IhcMi to go on 

 increasing', till after 1 I yeai's the maximum is reached again. 



With that object we again refei' to oui- spiral and start from the 

 14 con\olutions of (he year of S})Ot maximum. 



The second year's spiral is sliglitly shifted with respect to that of 

 the first, l)ut it runs still \ei'v close to it and therefore its position 

 with regard to the system of surfaces of discontinuity will resemble 

 that of the lirst, which was the most favoiirahlc for the observation 

 of spots; the]ice we may conclude that the dijference l)etween the 

 number of spots seen in the lirst and second year will be but small. 



The spiral of the third year (li^'erges again from that of the second 

 and is consequently somewhat farther removed from the spii-al, which 

 traversed the series of optical combinations nH)st favourable to the 

 observation of spots, and so on. 



We must not lose sight of the fact that the track of /-• has but 

 a slight inclination with respect to the surfaces of discontimiity, so 

 that at one time the Earth may remain for a rather long while 

 under the iiifluence of such surfaces, at other times, during nmch 

 longer periods still, may pass between them. 



There is no reason to expect that the decrease in the iiumber of 

 spots seen will |)roceed in a perfectly regular maimer, but at all events 

 there must be a year-spiral in which the circumstances ai'e most 

 unfa\oui'able for their ol>servatio]i ; for in proportion as the twelfth 

 spiral is a{)i)roaclied, which nearly coincides with the first, the con- 

 ditions must necessarily again improve. 



The length of the spot |)ei'iod is ii-regidar and the height of the 

 maxima varying. This woidd already be the case, were the Sun 

 totally stationary, for the twelfth s|)ii-al (d" /''does not exactly coimude 

 with the first; l)esides it is evident that modifications in the distri- 

 bution of matter may cause even greater irregnlai-ities in the successive 

 fluctuations. 



2. Prominences. 



On a foi'iner occasion ') we ha\e already given an explanation of 

 the ai)pearance of pnmiinences and their spectral i)eculiarilies, based 

 on our liy|>otliesis that they are due to the anomalous dispersion of 

 the photosphere light in the whirling parts of the surfaces of discon- 



1) Proc. Roy. Acad. Amst. IV, p. 162. 



