54 SCIENCE PROGRESS 



in ultra-violet rays. This is specially true of arc lamps in 

 which the carbons are impregnated with metals or have a metal 

 core. It is true in less, but still marked degree, of the various 

 metallic filament lamps — osram, tantalum, etc. It is also true 

 of Nernst lamps. Schanz and Stockhausen have published some 

 very interesting observations on the spectra obtained from these 

 lamps photographed with a quartz train. The results show the 

 effect of various kinds of globe upon the spectra. Ordinary 

 clear glass globes cut off all the shorter wave-length ultra-violet 

 rays — i.e. beyond 300 fifi. Those from 400 fj.fi to 3001111 pass 

 unimpeded. They have succeeded in obtaining a glass, called 

 " Euphos " glass, which cuts off all the ultra-violet rays without 

 much loss of the luminous rays. Mercury vapour lamps emit 

 light which is very deficient in luminous rays at the red end 

 of the spectrum but rich in those towards and beyond the 

 violet end. For some purposes, e.g. photography, therapeutics, 

 etc., it is desirable to utilise the ultra-violet rays to the full. 

 For this purpose another kind of glass has been invented, 

 which resembles though it is not so efficient as quartz. It 

 is called " Uviol " glass. The most powerful ultra-violet lamp 

 is a mercury vapour lamp in which the tube is made of quartz 

 and in which the mercury vapour is under pressure. By in- 

 creasing the pressure the spectrum obtained, instead of con- 

 sisting of isolated lines or groups of lines, becomes continuous. 

 Great heat, however, is developed b} r these lamps, necessitating 

 their being cooled by a water-jacket. 



It will be seen, therefore, that the ordinary lamps of com- 

 merce when protected by glass globes emit ultra-violet rays 

 of from 400 fifi to 300 iifi only in any considerable quantity. It 

 is not yet known whether these rays are more or less deleterious 

 to the eyes than those of shorter wave-length than 30011/1. It 

 is certain that they are deleterious when concentrated and 

 when the exposure is prolonged. It is to be hoped, therefore, 

 that some such glass as the new " Euphos " may be employed 

 for these lamps whenever they are used simply for the purpose 

 of illumination. There is obviously less danger in the use of 

 even naked arc lamps, since their heat and candle-power are so 

 great that they are little adapted for indoor illumination. 



