358 



On a New Cadmium 



Vapour Arc Lamp. 



Tabl 



E I. 



Visible Spectrum o£ Gallium. 



Wave-length 

 A. 



Intensity. 



3020-61 



3 



4033-18 



10 



4172-22 



20 



6396-99 



8 



6413-92 



6 



When the lamp is operated at a temperature sufficiently 

 high to bring the quartz to a cherry-red colour, and there is 

 danger of softening the lamp, several gallium lines become 

 faintly visible. The investigations of Uhler and Browning* 

 indicate the possibility of two gallium lines 5353*81 A and 

 5359'8 A. However, these lines, if present, are so faint at 

 the highest temperature at which the lamp can be operated, 

 that they cannot be identified. The cadmium spectrum is 

 thus obtained in a condition exceedingly favourable for 

 those purposes for which an intense monochromatic light 

 source is indispensable. No gallium lines are found between 

 4200 A and 6400 A, and the gallium lines which are detect- 

 able have so low an intensity that they are wholly negligible 

 in polarimetric and other fields of work. 



There are now available practically no dependable intense 

 monochromatic red light sources. Any source to meet 

 modern demands must permit of continuous operation with 

 minimum amount of attention and an absence of flicker. 

 The very pure red line (X — 6439 A) of cadmium seems to be 

 the only possible source of sufficient intensity in this region 

 of the spectrum. It is believed that the cadmium-gallium 

 lamp will make this much needed source, as well as other 

 lines of the cadmium spectrum, available for many lines of 

 endeavour. 



The writer desires to acknowledge his indebtedness to 

 Mr. F. P. Phelps for valuable assistance in the experimental 

 work. 



Bureau of Standards, 

 Washington, DC. 



* Amer. Jour. Sci. xlii. p. 389 (1916). 



