CH. XIII] RADIANT EFFICIENCY OF ARC LAMPS 



567 



appears in the form of radiation visible to the eye as light. A large 

 amount of energy is radiated in the form of ether waves of such 

 great length that they do not effect the eye and are called infra-red 

 radiation. A small amount of energy is radiated in the form of 

 very short invisible waves capable of exciting fluorescence and 

 affecting a photographic plate, this is called ultra-violet (fig. 307). 



RADIANT EFFICIENCY OF ARC LAMPS 



775, 776. In 1911 some experiments were made to determine 

 the entire energy radiated by the arc, and the relation of this energy 

 to the visible part of the radiation ( 776a). 



Briefly, the method consisted in getting side by side two patches 

 of light, which are photometrically equal. One of these patches 



FIG. 308. ARRANGEMENT OF APPARATUS TO MEASURE L'/R. 

 (From the Physical Review). 



Energy from the source L can reach the thermo- junction of the radiomi- 

 crometer Ra by either of two paths, (a) direct, no absorption except by air, 

 (b) through the prism train P. 



Light from the source is focused by the condenser C x on the adjustable slit 

 S It is rendered parallel by the lens C 2 , dispersed by the prism P and focused as 

 a spectrum R-Vby the mirror M T . The screen S 2 is placed in the red end of the 

 spectrum so that it cuts off all of the infra-red to .68/n. The mirror M 2 reassem- 

 bles the spectrum to a patch of white light at the radiomicrometer. 



The intensity of the patch of direct light is fixed by the brightness and dis- 

 tance of the source L, but that of the other patch Wean be varied by widening 

 or narrowing the slit 5, until it is of the same brightness as the direct light. 



The prism consists in a 60 hollow prism of carbon bisulfide immersed in a 

 square glass cell filled with distilled water. It gives a good dispersion with a 

 deviation of but 20 from a straight line. 



The lenses are of glass. The mirrors are plano-concave lenses, silvered on 

 the concave side. The focal length of JW, is 50 cm. and of M z is 25. 



