MERCURY VAPOUR LAMPS 



93 



simpler matter to cut off the few superfluous bands and 

 leave light of one wave length, than to find a filter which 

 will pass rays of one wave length only from light having 

 a continuous spectrum. The vapour of mercury has been 

 found most convenient for this purpose, and lamps are 

 made consisting of a highly exhausted glass tube, con- 

 taining a small quantity of mercury which is vaporised on 

 the passage of the current, forming a conductor between 

 the two poles and glowing with considerable intensity. 



Fig. 27 



THE HAGEH MERCURY VAPOUR LAMP 



The intensity of the monochromatic light obtained from 

 the mercury vapour lamp by the use of suitable light 

 filters is sufficient for high powers, but unfortunately the 

 tubes are rather fragile and expensive to replace, and its 

 use has so far been rather limited on that account. Tubes 

 made of quartz are sometimes used ; they are better in 

 wearing quality than those of glass, but are more expensive. 

 Fig. 27 is an illustration of the Hageh mercury vapour 

 lamp made by Zeiss. The tube is about 20 cm. long 

 and carried on an adjustable support. Flasks of some 



