CH.VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 183 



the right-angled arc goes out easily from the magnetic blow when 

 the current is above 25 to 30 amperes. Of course, in opaque pro- 

 jection, where the most powerful light available is demanded, 

 alternating current is far less satisfactory than direct current ; still 

 with skillful application of the light available even alternating 



FIG. 99. THE INDEPENDENCE POST-CARD PROJECTOR. 

 (Cut loaned by Williams Brown & Earle). 



This is in principle exactly like Chadburn's opaque lantern with two lamps 

 (fig- 93)- I n this projector the lamps are usually of the incandescent form, and 

 connection is made with the house-electric lighting system. 



current radiants give fairly good opaque projection (see Ch. XIII, 

 7 53 a for size of carbons with different currents, etc.). 



For favorable objects and good conditions one must use not less 

 than 20 to 25 amperes of direct current for successful screen pic- 

 tures of opaque objects. Those with most experience in the work 

 use 40 to 50 amperes. 



For alternating current satisfactory results can hardly be 

 obtained with less than 40 amperes, and 60 to 80 are better. 



