174 



BIOLOGICAL EFFECTS OF RADIATION 



Contrast with this the high altitude records for Montezuma, plotted 

 for the small precipitable water 0.05 cm. in Fig. 21 (4). 



4.000 6,000 8,000 10.000 12,000- 14,000 16.000 18,000 20,000 



Fig. 20. — Spectral distribution of radiation from the sun, outside the atmosphere, and 

 at air masses 1, 2, 3, 4, and 5, on an average day in Washington, precipitable water 1.37 cm. 

 Dotted curves indicate distribution from artificial sources, i.e., radiation from 100-watt 

 tungsten lamp, at approximately 2755° K., same through 3-cm. water cell, through 3-cm. 

 water cell and the following filters (all heat-resisting, heat-absorbing) : extra light shade, 

 2.75 mm. (thickness); light shade, 2.66 mm.; light medium shade, 2.62 mm.; dark medium 

 shade, 2.97 mm.; dark shade, 2.82 mm. (From data of Abbot, 4.) 



These curves do not give any idea of the wave-length distribution 

 below 4000 A, i.e., in the ultra-violet. From 1 to 5 per cent of the energy 

 occurs in this region. A typical distribution curve of relative intensity 

 for wave-lengths between 2900 and 3600 A is plotted from data of 



