CH. VII] TROUBLES IN OPAQUE PROJECTION 197 



2. If the amperage is to exceed 25 or 30, it is better to use an arc 



lamp with inclined or vertical carbons, not those at right 

 angles for the magnetic blow puts the right-angled arc out 

 too easily. 



3. Do not have the carbons in contact with a hand-feed lamp 



when the current is turned on. Feed them together after 

 the current is on, then they can be separated properly 

 immediately after the arc is struck. 



4. Inverted screen image. The object not properly placed on 

 { the support, or no erecting mirror or prism is used. 



5. No detail in the screen image. The object may be too light- 



absorbing, or the light may not be sufficient. 

 (See Troubles in Ch. I.). 



