Ultraviolet Light 231 



that occur in both plants and animals. The relative effect of different 

 wavelengths in producing the foregoing influences is shown in Fig. 

 6.25. Although the curves differ very much from one another 

 throughout most of the range, it is clear that in no instance is ultra- 

 violet radiation effective at wavelengths longer than about 3100 A. 



240 



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300 



310 



320 



rriju 



270 280 290 



Wavelength 

 Fig. 6.25. Relation between wavelength in the ultraviolet region and the bac- 

 tericidal, erythemic, and antirachitic effects of ultraviolet radiation, together with 

 the absorption of ergosterol. (Coblentz, 1939, Copyright, J. Am. Medical Assoc.) 



Under ordinary conditions no measurable radiation from the sun 

 reaches the earth's surface at wavelengths shorter than 2950 A. It is 

 clear then that under natural conditions the whole action of ultraviolet 

 must occur between 3100 A and 2950 A. Under the best circum- 

 stances solar radiation is extremely weak in this region, and when dust 

 or smoke is abundant the shorter wavelengths are still further re- 



