SUNLIGHT AS A SOURCE OF UADIATIOX 97 



that radiations Avhirh emerge from the edge of the spherical sun come 

 from higher and cooler levels than the radiations from the center of the 

 disk which must pass through minimal thickness of overlying gases in 

 escaping. Since the attenuation by scattering and absorption is greater 

 for short than for long wave lengths, the effect is more pronounced in the 

 blue than in the red end of the spectrum. 



Although the sun is entirely gaseous, the sharpness of the photosphere, 

 in comparison with more nebulous layers above it, has led to the custom 

 of referring to these latter regions as the atmosphere of the sun. There 

 are three regions of the solar atmosphere distinguishable by the states in 



Fig. 3-1. Ultraviolet solar spectruiu ol)tained in 1942 at Arosa, Switzerland. {Gotz 

 and Caspar is, 1942.) 



which matter exists in them and by the radiations which they emit — the 

 reversing layer, the chromosphere, and the corona. The reversing layer 

 is the innermost of these regions and lies just above the photosphere. 

 It extends to a height of 1500 km above the photosphere and is the region 

 in which the transition from continuous to line emission occurs. The 

 temperature is about 4830°K, and the pressure is probably 10"^ to 10~^ 

 atm. In the reversing layer the dark Fraunhofer lines of the solar spec- 

 trum are formed by atomic absorption at discrete wave lengths of the 

 continuous radiation from the underlying hotter photosphere. The ultra- 

 violet spectra of Figs. 3-1 and 2 show many of the large number of 

 Fraunhofer lines in the biologically effective region of the solar spectrum. 

 Babcock et al. (1948) have investigated the ultraviolet solar spectrum 

 between 2935 and 3050 A with a 21 -ft grating spectrograph and have 

 listed 665 absorption lines in this erythemal region. 



It must not be supposed that the dark lines represent points of zero 

 intensity in the solar spectrum; they appear so in Figs. 3-1 and 2 because 



