28 PROCEEDINGS OF THE AMERICAN ACADEMY. 



A the distribution of energy in the directly received solar light. 

 Curve B shows the distribution of energy in the diffused light of the 

 blue sky when the total of this diffused energy equals 20 % of the 

 total directly received solar energy, a not uncommon proportion. It 

 w ill be noted that the maximum for this curve is in the far violet near 

 the edge of the ultra violet. Curve C is the summation of A and B 

 and it will be seen at once that the proportion of ultra violet is some- 

 thing like three times as great as in the case of the direct solar rays. 

 This proportion would raise the ultra violet activity of daylight to 

 a point higher per foot candle than that reached by any ordinary 

 artificial illuminant. 



Weisner -^ in photographic observations of light received on hori- 

 zontal surfaces states for example, "For solar altitudes less than 

 19 degrees the chemical intensity of the sunlight as compared with 

 diffused daylight is negligil)le, with increasing solar altitude it gains 

 in comparison with the diffused daylight. * * * Since the intensity 

 of the direct beam may reach twice that of the diffused, the total 

 combined chemical effect may be three-fold that of the diffused light." 



Daylight, therefore, varies very greatl}- in ultra violet energy, rang- 

 ing from the low value given in Table III for direct sunlight to values 

 which would exceed almost all artificial light sources. The chief claim 

 of simlight to serious consideration from the standpoint of ultra violet 

 energy, howcAer, lies in the very large amount of energy which the 

 sun delivers. There is considerable doubt as to the exact amount of 

 solar radiation outside of the atmosphere, but that which gets through 

 the atmosphere is pretty well determined and its amoimt, from the 

 data given by Abbott -- amounts practically, under favorable condi- 

 tions, to not less than 1 kw. per sc^uare meter, which is 0.1 watt per 

 square cm. If one assumes that only 10 % of this is in the ultra violet 

 region, an amount which may be exceeded at times, the total ultra 

 violet radiation rises to 10^ ergs per second per square cm., several 

 times that given by the most powerful artificial sources of ultra violet 

 at even a distance of so short as half a meter. 



Considering this very large flux of ultra violet energy it is small 

 wonder that troubles from sunburn and snow-blindness are not 

 infrequent. Did we not habitually shield our eyes by interposing the 

 rim of the hat or the brow and by systematically looking away from 

 the direct sunlight ocular troubles would be common and severe. 



21 Denkschriften Wien. Akad., Vol. 64, 1897. 



22 "The Sun". Chapter VII. 



