the Constitution of the Sun. 97 



Having stated the theory, we will now apply it to the four 

 classes of phenomena defined above. 



From the nature of the condensation, the granules or cloudy 

 masses must be very transparent, because the condensation is 

 confined to elements which have very high boiling-points, and 

 because such elements can be but a portion, perhaps but a 

 small portion, of the whole matter contained in the upward 

 currents. 



It is not a priori improbable that we receive light from 

 many hundreds of miles below the general outer surface of the 

 photosphere. Since these cloud-like sources of intenser radia- 

 tion are surrounded on all sides by descending currents of 

 colder vapours, all the white light which comes to us must 

 have passed through media capable of modifying it by selective 

 absorption. Again, since at the centre of the solar disk we 

 can see as far into the photosphere as at the limb and practi- 

 cally no further, the phenomena of absorption ought to be, on 

 the whole, the same in both regions. 



Thus the fundamental and most important class of pheno- 

 mena above classified finds a simple and logical explanation. 



With regard to the phenomena of class II., we have but to 

 define the problem in order to find the solution at hand. All 

 the lines of class II. belong to vapours which lie high in the 

 solar atmosphere, as is evident from their frequent reversal in 

 the chromosphere. On the centre of the disk these lines are 

 hazy or "winged," but not so at the limb. To the spectro- 

 scopist this aspect is characteristic of greater pressure, that is 

 of more frequent molecular impact. The observation, then, 

 proves that the dark lines of hydrogen, magnesium, sodium, 

 &c, as seen at the centre of the solar disk, are produced by the 

 elements in question at a higher pressure than the correspond- 

 ing lines at the limb. Accepting our theory, this must be so; 

 for supposing the transparency of the photosphere is such that 

 we can see into it a distance of 2000 miles, then at the centre 

 of the disk we have light modified by selective absorption all 

 the way from the extreme outer chromosphere down to 2000 

 miles below the upper level of the photosphere ; while 10" 

 from the limb the light, though coming from the same depth 

 of vapour measured along the line of vision, has its lowest 

 origin more than 1700 miles further from the sun's centre than 

 in the previous case. Of course the numbers here used have 

 no definite significance ; but modify them as Ave will within 

 the bounds of probability, the reasoning remains the same. 



Suppose now a certain vapour which is confined to the 

 upper stratum of the photosphere, or rather one of which the 

 lower limit is thus restricted ; then, according to the reason- 



Phil Mag. S. 5. Vol. 11. No. W.' Feb. 1881. H 



