176 



BIOLOGICAL EFFECTS OF RADIATION 



in this region, the type of long-wave cut-off employed will drastically 

 hifluence the values obtained. 



1250 



4000 



4500 



5000 



6000 



6500 



5500 

 WAVE LENGTH 



Fig. 22. — Sky radiation, intensity in arbitrary units. {Frojn data of Abbot, 1.) 



7OO0 A 



9 10 11 12 I 

 Hour 



Fig. 23. — Ultra-violet radiation on clear days. {Zinc sulfide measurements by Clark, 18.) 



The contribution of sky radiation is much greater for short wave- 

 lengths, as will be seen from Fig. 22, plotted from Abbot's data (1). 

 The sky radiation in the region 2900 to 3200 A often exceeds the direct 

 radiation even for high sun. 



