PRESENT STATE OF THE PROBLEM 65 



only manifest itself in the presence of water and oxygen 

 (these are absent in outer space) ; that the intensity of the 

 radiations was less in space than in the experiments ; that 

 ultraviolet radiation was not effective at temperatures near 

 to absolute zero, etc. 



120 



2500 3000 



WAVE LENGTH, A 



3500 



Fig. 5. Ultraviolet portion of the solar spectrum on 

 top of the atmosphere. 



By permission from Radiation Biology, vol. 11 by 



A. Hollaender. Copyright 1955, McGraw-Hill Book 



Company, Inc. 



These objections, however, did not stand up to strict 

 experimental testing. At the present time direct experi- 

 ments using rockets which are sent up to heights of loo 

 kilometres, i.e. considerably above the ozone screen, have 

 shown that, at this level, ultraviolet radiation is far more 



