( 275 ) 



^ 2,25 

 of ^77-^77^ X '^ ^- '^'''<^' spirals ol jlio Iwcltdi and lliii'lcenlli voar 



come again very close to that of the first year and lie on each side 

 of it. It is therefore reasonable to expect that after a period of 

 a little more than 11 years a very similar snccession of incidents 

 will take place. ^) 



It now remains for us to consider which conditions and phenomena 

 may be governed by the i)osition of /-* on the sphere. 



The vSun is an immense mass of matter, and considering its age 

 we ma)' take it for granted, that within the memory of man it has 

 remained in an almost stationary condition. We Ivnow that the 

 violent ernptiuns which have been thought to take pkice on its sur- 

 face, have led to a quite ditferent conception, but at present we can 

 realise ^) that relatively small local variations iji density, such as 

 necessarily must occur in vortices along the surfaces of discontinuity 

 l)etween stationarily streaming layers of gas, are quite sufticient to 

 produce strongly marked variable optical effects, such as promi- 

 nences etc. 



The large currents of the general solar circulation must be 

 cyclic movements, which do not perceptibly alter the configuration 

 of the entire mass, only causing along the surfaces of discontinuity 

 a somewhat xaryiiig distribution of matter, due to undulations and 

 whirling. We admit that on account of the Sun not being perfectly 

 symmetrical around its axis, the mo^■ability of the parts involves a 

 gradual change of form, but this change we will leave for the 

 present out of consideration. 



The rays emanating from the intensily bright core of the Sun 

 reach us, whatever be the position of the Earth, through a space in 

 which matter is unequally distrilnited. P therefore determines the 

 principal characteristics of what might be called the "optical system" 

 through which we see the Sun. When P shifts its position, this 

 system changes with it; when P for the second time traverses the 

 same path on the rotating sphere, to die eye of the observer on Earth 

 all the phenomena which are produced by the refraction of light in 

 the gases of the Sun, will repeat themselves in the same order. 



^) Had we for the Hornstein period taken 25,9:24 days, instead of 25,920, 



25,924 

 then tlie mean value of the spot period would liave been — '~ — = 11.17 years. If 



therefore we wished to supplement the table on p. 272—273 with a value derived 

 by theoretical considerations from the LI -year period, the number 25,924 would 

 commend itself. 



2) W. H. Julius, Proc. Koy. Acad. Amst. IV, p. 1(32—171. 



