96 RADIATION" HlOI.onv 



sharply deliiiccl of these t raiisitioiuil regions. When viewed in visible 

 light the photosphere appears to ))e a sharply defined disk, 1.39 X 10^ km 

 in diameter, considerably brighter at the center than at the edge, or limb. 

 It is the source of the continuous spectrum of the sun, or rather, the re}>;i()n 

 in the sun in which predominantly contiiuious emission changes rather 

 abruptly into emission of spectral lines. This transition, owing to a 

 rapid decrease in opacity of the solar gases in the region of the photo- 

 sphere, takes place in a few hundred kilometers in a level where the pres- 

 sure is about 10~' atmosphere. 



The transmission coefficient tx of a homogeneous layer of absorbing gas 

 for light of wave length X is given by 



h = r-'-x" (3-1) 



Avhere «x = the absorption coetficient for wave length X and h = the 

 thickness of the absor))ing layer. The sharpness of the plnjtosphere 

 depends on the circumstance that, for visible light, the values of a\h are 

 sufficiently high that a relatively thin layer h of the gases is opaque; that 

 is, the i)roduct a^h is very great. Relative to any comparison wave 

 length, say, in the visible region, radiation emerging from the sun at other 

 wave lengths where ax is greater will come from higher and therefore 

 cooler layers, and the intensity will be lower. At wave lengths where ax 

 is smaller, a thicker layer of the sun is required to be opaque, and the 

 radiations which escape come from a region of higher temperature and 

 are, accordingly, relatively more intense. Thus a part of the visible and 

 near-infrared continuum of the photosphere matches the spectrum of a 

 ()000°K black body rather well, whereas the blue and ultraviolet intensi- 

 ties lie considerably below a (i()00°K source, and there is evidence that the 

 infrared spectrum in the 8- to 13-ju region is fitted better by a 7000° K 

 intensity curve. 



Although the photosphere is sharp in visible light, it is not uniformly 

 l)right along the diameter, the center of the disk being considerably 

 brighter than the edge. This phenomenon, readily observable with the 

 eye or in photographs of the sun, was examined by Abbott et al. (1922) at 

 seven wave lengths between 3737 and 10.080 A. Considering the inten- 

 sity at the center of the disk to be unity for each wave length, it was 

 found that the intensity decreased toward the edge of the sun for all 

 wave lengths and that the diminution in intensity was more pronounced 

 for short than for long wave lengths. For example, at a distance from 

 the center equal to 0.95 of the photospheric radius, the intensity of wave 

 length 3737 A was 0.4319 of the central intensity, whereas at 10,080 A it 

 was 0.7331 of the central intensity. Thus the limb of the sun is not only 

 less bright but is also redder than the center. The effect is explained by 

 the considerations in the foregoing discussion and is probably due both 

 to absorption and scattering by overlying layers of gases and to the fact 



