i6o 



THE ART OF PROJECTING. 



by the movement of the upper carbon alone, and this 

 will not answer at all for lantern work. Both carbons 

 must move. The annexed cuts represent electric arc 



lamps designed to do this. 

 Marcy has also an electric 

 lamp in which the upper car- 

 bon is inclined so as to 

 present the concave surface 

 of the glowing carbon to the 

 condense^ which device ap- 

 pears to work well. A good 

 electric arc gives light equal 

 to about a thousand stand- 

 ard candles, while very 

 ordinary ones give five or 

 seven hundred. For most 

 teachers' uses, however, the 

 steady projection of trans- 

 parencies is seldom needed, 

 but an electric light for 

 common purposes that may 

 be had by simply turning a 

 switch is highly desirable. 

 There is said to be an ad- 

 vantage to be derived from 

 combining the arc light with 

 QUEEN'S ELECTRIC LAMP. the incandescence of lime, 

 the latter giving a degree of steadiness and a brighter 

 light with a given current than would be had without 

 it. The block of lime has a hole through it large enough 

 to allow the carbons to move loosely in it. Near the 

 middle of the block, on one side, a hole is cut through 

 to meet the other, and it is opposite to this hole that 

 the carbons are to touch and the arc be formed, shin- 



