266 



radius; and the law of' variation of' brightness with the angle is 

 different for different wave-lengtlis. 



Wl»i('liever the causes may be, that make tlie sun radiate irsore 

 intensely in the direction of the radius than in (Hrections slanting 

 to it, they must be looked for in layers lying ^f/o^y the level generally 

 called the surface of the photosphei-e. Those layers consist of trans- 

 parent gases, for Ihe slightest haze of condensation prodncts, occupying 

 a stratum some (housand kilometers thick, would provide it with a 

 radiating and scatteririg power almost independent of direction, which 

 power the photosphere does not possess. 



Assuming, on the basis of the Maastricht j-esults, that the extinction 

 effected by the sun's outer layers is comparatively small, we derive 

 from direct observations on the distribution of l)i-ightness on the 

 sun's disk (Voget., Abbot), how much light of a gi\en wave-lenglh 

 a point Jf, lying somewhere in the photospheric level, transmits on 

 the average along the various directions. The result may conveniently 

 be described, for c\ery wave-length separately, by means of an 

 "irradiation surface" q f> q^ (Fig. 1)'), the radii vectorcs of which 



represent the average intensitiec of the light reaching .1/ from 

 different sides. We obtain the "eradiation or emission surface" p // q^ 

 of M by prolonging the radii rM and making Mr' = rM. 



If we wish to explain the sun's api)arent, fairly sharp, boundary, 

 and the law of varying brightness of the solar disk, we shall have 

 to consider, besides emission and absorption, the effects of dispersion, 

 refraction, and molecular scattering of the light traversing an entirely 



1) For a method of constructing these surfaces we refer to these Proceedings 

 XIU, 1263; or: Physik. Zeitschr. 12, 677, 1911, or: Handwörterbuch der Natur- 

 wissenschaften, VII. 830, (1912). 



