146 PROCEEDINGS OF THE AMERICAN ACADEMY 



In our theory, then, the granules are those portions of upward cur- 

 rents where precipitation is most active, while the darker portions, 

 between these bodies, are where the cooler products of this change 

 with accompanying vapors are sinking to lower levels. 



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 ele- 

 ments can be but a portion, perhaps but a small portion, of the whole 

 matter contained in the upward currents. It is not a priori improb- 

 able that we receive light from many hundreds of miles below the 

 general outer surface of the photosphere. Since these cloud-like 

 sources of intenser radiations are surrounded on all sides by descend- 

 ing currents of colder vapors, all the white light which comes to us 

 must have passed through media cajDable 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 practically no farther, the 

 phenomena of absorption ought to be, on the whole, the same in both 

 regions. 



Thus the fundamental and most important class of phenomena 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 vapors 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 spectroscopist 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, etc., 

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

 in question at a higher pressure than the corresponding lines at the 

 limb. Accepting our theory, this must be so ; for, supposing the trans- 

 parency of the photosphere is such that we can see into it a distance 

 of 2,000 miles, then at the centre of the disk we have light modified 

 by selective absorption all the way from the extreme outer chromo- 

 sphere down to 2,000 miles below the upper level of the photosphere ; 

 while 10" from the limb the light, though coming from the same depth 

 of vapor measured along the line of vision, has its lowest origin more 

 than 1,700 miles farther from the sun's centre than in the previous 

 case. Of course the numbers here used have no definite significance, 



