160 The Blue Sky and the Optical Properties of Air [May 7, 



show that the mercury lamp spectrum was by no means stopped when 

 the solar spectrum stops, but that it extended to the region where 

 ozone is most opaque. There is a strong mercury line (wave-length 

 2536) at about this point which was distinctly photographed. Its 

 intensity was of course a good deal reduced relative to the visible 

 spectrum by atmospheric scattering. But there was no evidence 

 whatever of ozone absorption. 



What conclusion can we draw ? Evidently that the absorbent 

 layer of ozone in the air is high up, and that there is little or none 

 near the ground. It may seem at first sight that this thin and 

 inaccessible layer of ozone, of which we have learned by a chain of 

 reasoning not less conclusive than direct observation, is a matter 

 of little importance to man and his welfare. There could be no 

 greater mistake. It acts as a screen to protect us from the ultra-violet 

 rays of the sun, which without such a protection would probably be 

 fatal to our eyesight, at least if one may judge from the painful 

 results of even a short exposure to such rays, which those who have 

 experienced it are not likelv to forget. 



[R.] 



WEEKLY EVENING MEETING, 



Friday, May 14, 1920. 



Sir James Crichton-Browne, M.D. LL.D. D.Sc. F.R.S., 

 Treasurer and Vice-President, in the Chair. 



Karl Pearson, F.R.S. 

 Sidelights on the Evolution of Man. 



[For Eeport of Lecture see University of London Galton Laboratory 

 for Natural Eugenics Lectures, Series XIII. (1921)]. 



