18 PROCEEDINGS OF THE AMERICAN ACADEMY. 



abandoned and the readings taken with the globe on, which proved 

 reasonably easy, the precision being comparable with that obtained 

 with the ordinary incandescent lamps. But even then the lamp 

 proved very sensitive to small changes of voltage and only by Aery 

 careful regulation of the current could consecutive series of readings 

 be held in reasonably close agreement. 



In the average the deflection due to the ultra violet in the Nernst 

 lamp with its globe was 1.81 cm. and the percentage of energy thus 

 cut off was 5.2. This completed the radiometric investigation of 

 ordinary illuminants. Two others which it seemed desirable to 

 investigate, that is the ordinary flame arc, and the arc between iron 

 electrodes as used by Finsen were studied on the spectrograph, since 

 their fluctuations were of a character to make their study by means 

 of a galvanometer of so long period as that used in this investigation 

 quite impracticable. The peculiarities of these sources will be referred 

 to in discussion of the general results. 



Szm Light. — Finally it seemed advisable to take some comparative 

 readings on sunlight as a source of ultra violet radiations, particularly 

 with reference to the amount of ultra violet energy with respect to the 

 intensity of the light. Of course the solar radiation in absolute amount 

 has been investigated with great thoroughness, but the ultra violet 

 has received less attention than the rest of the spectrum. In general 

 the sun radiates energy substantially like an incandescent black body 

 at about 6000 degrees C. except in so far as its energy, particularly 

 in the ultra violet, is cut off by the absorption of its own and the 

 terrestrial atmosphere. It behaves then, like an enormously hot 

 incandescent body shining through a medium that cuts off all the 

 ultra violet of less wave length than about 295 MM and greatly dimin- 

 ishes the shorter radiations even into the violet of the visible spec- 

 trum. One woidd expect therefore to find relatively little total ultra 

 violet per unit of illumination so far as the direct light of the sun is 

 concerned. On the other hand as Schuster '^ and others have shown, 

 much of this cutting oft' of the ultra violet is due to scattering of the 

 short waves by the molecules of the atmosphere and. small bodies 

 suspended in it. In other words, the violet and ultra violet are not 

 wholly lost, but appear in radiation from the blue sky. 



Of the energy thus radiated from the sky the maximum lies almost 

 in the edge of the ultra violet. The arrangement of the apparatus 

 for experiments on sunlight is shown in Figure 5. Through the 



'^Nature, XXXI, p. 97. 



