88 THE ELECTRIC ARC 



so concentrated as in other forms. Practically all of the 

 light comes from the vapor, and this is usually several 

 inches in length and about one inch in diameter. Again 

 it has a high efficiency and does not require any attention. 

 There is no occasion to renew electrodes nor to remove 

 products of combustion. As long as the tube keeps out 

 the air it is in working order and manufacturers claim a life 

 of 5000 hours for the lamp under favorable conditions. 

 Lastly it does not require any elaborated feeding mechanism. 

 Nothing could be simpler than the Cooper Hewitt form of 

 this lamp. 



On the other hand it has the disadvantage of giving an 

 unpleasant light. It is not so much that the light appears 

 to one looking at it to be different from white light, as that 

 it gives the wrong color to objects which it illuminates. 

 This is due to the absence from its spectrum of some im- 

 portant colors. Its spectrum is a line spectrum in which 

 only a few lines are prominent. It is lacking in red, while 

 the green is very prominent. As a result it gives to the 

 human face, for example, a disagreeable and lifeless appear- 

 ance. The lamp is, however, of much value in certain 

 special uses, as in a drafting room, where only the distinc- 

 tion between black and white is desired, or in photographic 

 work, where the strong violet rays are of use. 



The first arc between mercury terminals of which we 

 have any record was one described by Wey. 1 This, how- 

 ever, was an arc in the air and showed nothing of especial 

 interest. 



The first mercury arc in a vacuum was described by 

 Arons in 1892. 2 The arc was in an inverted U-shaped glass 



1 Dingler's Polytechnic. Journ., 157, 399; 1860, and 159, 46; 1861. 



2 Wied. Ann., 47, 767, 1892. 



