271 



of that order of ina-uitihlc. In a layer, for instance, where (lie 

 average density dees not exceed (he density of our own atmosphere 

 at sea-levei, a leniperature gradient of J°.4 C. per kilometer is all 

 that would be required. 



§ 3. The above dioptrical conception of the photosphere implies 

 the following explanation of the variation of brightness across the disk. 



This problem, indeed, may also bo ex])ressed as follows: what is 

 the cause of the fact, that the irradiation surface of a point 1/, lying 

 somewhere in or near the "photospheric level", has that particular 

 shape (difFerent according to the selected wave-length), which direct 

 observation assigns to it? 



Let PP' (Fig. 3) represent a part of the photospheric level, CO 



r. 



M 



M 



of another level lying so much deeper, that there the solar matter 

 is dense enough to emit light giving a continuous spectrum. 



Although the medium surrounding M be a mixture of selectively 

 absorbing gases, transparent to the greater part of the spectrum, 

 that transparency is not absolute, ^foli'cular scaiteriw/ (Rayleigh) ') 

 Aveakens a direct beam according to the \a\\' 



P.-' 



Ill winch .'; 



32-1 ='(// — 1)- 



but if the source of light be an incandescent surface CC' , radialiny- 

 the energy /.. per square unit, and if the dilfused light itself be 

 taken into consideration, the energy emerging per square unit fiom 

 PP' will (as found by Schüstkr) ') be expressed thus: 



2 



/= /. 



') Rayleigh, Phil. Mag. [5] 47, 37ö, (1899). 

 '^) Schuster, Aslcopli. Jouin. 21. 1, (190;j). 



Abbot, in his valuable book "The .^un", HOll), also introduces molecular scat- 

 tering as a principal agent in producing the appearance of the pliotosplierc. 



