90 



PHOTOMICROGRAPHY 



that the carbons are liable to give off a flame which 

 prevents a clear image of the crater being obtained. 



With alternating current, on the other hand, they 

 have a distinct advantage over the open arcs already 

 described, in that the arc is steadier and less given 

 to searching ; they are, however, still not suitable for 

 critical illumination. The greater length of time for 

 which they will burn without adjustment, when com- 



Fig. 25 



ENCLOSED ARC LAMP 



pared with an open arc, is not of such great importance 

 in photomicrography as for projection. 



Fig. 25 shows a very handy form made by the West- 

 minster Engineering Co., Ltd. 



If the Liliput lamp is used on an alternating circuit, 

 two carbons of equal diameter are used ; and in order 

 that the arc may be as steady as possible, the}^ should 

 be the smaller ones of 6 mm. diameter. The supply 

 wires are connected to the lamp-terminals indiscrimin- 

 ately. 





