178 



BIOLOGICAL EFFECTS OF RADIATION 



to 1 per cent transmission at 3000 A and 50 per cent or over at 3100 A. 

 From this crude estimation, we see that such a theory finds support in 

 the known characteristics of ozone absorption. While Dobson's observa- 

 tions have been obtained by ozone determinations in the ultra-violet, 

 Wulf (69) has shown that determinations by the visible ozone-absorption 

 band are in substantial agreement. The fact that ozone attains its least 

 effective depth well into July has been offered as the explanation for the 

 fact that the ultra-violet reaches a maximum intensity in July, whereas 

 the maximum total radiation occurs in June. The ozone theory of ultra- 





o. 



UJ 



Q 

 UJ 



> 



I- 

 u 



u. 

 u. 



lU 



Fig. 



FEB. MAP- APR. MAY JUNE JULY AUG. SEPT. OCT. 



25. — Seasonal variation in ozone. Effective depth in centimeters at standard con- 

 ditions. 



violet fluctuation has been severely attacked, however, by several 

 observers (61). 



The north-sky radiation, shown in Fig. 24, represents only half the 

 solid angle and less than half the sky radiation, as the scattering is greater 

 near the sun, which for this latitude generally occurs in the southern 

 hemisphere. 



o 



For the region between 2900 and 3130 A, Coblentz and Stair have 

 determined the values of ultra-violet radiation given in the curves plotted 

 by Laurens and shown in Fig. 26 (46, cf. page 68). (Values given in the 

 original publication (22) were increased 50 per cent on later calculations.) 

 Here it will be seen that the maximum amounts to 86 microwatts, as 

 compared with between 105 and 110 milliwatts maximum total radiation 

 for Washington, i.e., about 0.1 per cent. Clark's value for a somewhat 

 greater wave-length range shows a maximum of about 1 per cent. 



In summary, we may say that less than 0.1 per cent of the total 



o 



radiation lies short of 3130 A, 1 to 5 per cent lies in the ultra-violet, from 



