BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. -i 



distilled water in this thickness has any material absorption in even 

 the extreme ultra violet up to the limit investigated. 



The use of this cell therefore could produce no sensible effect on 

 the accuracy of the ultra violet measurements, while it did serve the 

 extremely useful purpose of limiting the total amount of energy to 

 be measured and of eliminating any difficulties that might arise owing 

 to absorption in the further part of the infra red, all the absorbing 

 media incidentally used being, as compared with water, practically 

 entirely transparent to all the radiations that got through the water 

 cell. It would have been convenient if some substance cutting off the 

 infra red sharply at 750 h/j, or 800 /i/x had been available. Unfortu- 

 nately, there is no such substance, so far as has yet been discovered, 

 the very few substances less transparent than water in the region 

 800 to 1300 /x/x being useless for the purpose of this investigation on 

 account of opacity in the ultra violet and generally in the visible 

 spectrum as well. Iron ammonium alum used by Lux (loc. cit.) 

 and the copper salts used by Fery (loc. cit.) are open to this objection 

 and the same is true of all the otherwise useful and promising sub- 

 stances discussed in the very thorough and valuable researches of 

 Coblentz.^'^ 



In some of the experiments a second similar c^uartz cell was used, 

 particularly in work on arc lamps. In this case the Euphos glass 

 used to cut out the ultra violet portion of the spectrum was perma- 

 nently affixed to one of these cells and either the plain quartz cell or 

 the Euphos-quartz cell was thrust into the beam so as quickly to get 

 differential readings. In order to avoid the somewhat large correc- 

 tion due to reflection of energy which would have been produced by 

 the introduction of a plain slip of Euphos glass to cut out the ultra 

 violet the following expedient was adopted. 



The Euphos glass was attached to the surface of the quartz cell by 

 spring clips with the addition of a thin capillary film of pure glycerine 

 between the quartz and glass surface. Glycerine is immensely trans- 

 parent to all radiations, including the extreme ultra violet, to which 

 Canada balsam and gelatine are quite opaque. Its index of refrac- 

 tion, 1.47 for D, is sufficiently near that for quartz and the various 

 glasses to reduce the loss of light at the surfaces to an entirely negligi- 

 ble amount. As the Euphos has a slightly less index of refraction 

 than quartz, there was a minute residual gain in the total transmis- 

 sion of the system when the Euphos glass was added, in the right 

 direction to compensate for the minute losses by absorption in the 

 glycerine film. 



11 Bull. Bureau of Standards, Vol. 2, p. 619. 



