BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 



27 



CO 



light given. In this connection the position of sunlight in Table III 

 is not without importance. On the face of the returns it has a less 

 amount of ultra violet with respect to the illumination than most of 

 the artificial illuminants. This is due to the very high temperature 

 of the source, which insures high luminous efficiency, in connection 

 with strong ultra violet absorption in the atmosphere. Unfortu- 

 nately one can apply Planck's law to very few practical sources 

 of light. The sun is ruled out by the very erratic and highly selective 

 absorption which produces the Frauenhofer lines and also by an 

 unknown absorption of the extreme ultra violet which may take place 

 in the earth's atmosphere or near the solar surface or in both places. 

 Incandescent lamps involve absorption by their globes and also in 

 the case of more recent ones a certain amount of selective radiation. 

 The whole tribe of arcs which yield in a greater or less degree discon- 

 tinuous spectra, for which Planck's law does not hold, are also thereby 

 eliminated, so that this 

 otherwise very useful 

 guide to the distribution 

 of radiation ceases to have 

 exact significance. 



The ultra violet com- 

 ponent of sunlight has 

 been considerably dis- 

 puted. It has been held 

 by some investigators like 

 Dr. Voege^^ that sunlight 

 contains more ultra violet 

 than the arc light, while 

 Schanz and Stockhausen -'^ 

 take the opposite view. 

 In a sense both are right 

 and both wrong. Sun- 

 light undoubtedly contains only a very modest proportion of ultra 

 violet per foot candle of illumination when one considers direct sun- 

 light alone. If, however, one considers the total daylight effect, 

 including skylight under favorable circumstances, the situation takes 

 on a totally different aspect. The light diffused by the blue sky is 

 mainly violet and ultra violet, being substantially that light of which 

 the direct sunbeam is robbed by scattering. Figure 6 shows in curve 



19 Tlae Illuminating Engineer, Lond., Vol. II, p. 205. 

 2" The Illuminating Engineer, Lond., Vol. I, p. 1049. 



40 



30 



20 



10 



300 -iOO 500 COO 700 800^i/i 



Figure 6. Curves of Sun and sky energy. 



