CH. VII] DO AND DO NOT IN OPAQUE PROJECTION 



199 



9. Use two radiants or mir- 

 rors for avoiding shadows with 

 solid objects ( 292). 



10. Select objects which re- 

 flect well for opaque projection 

 ( 285). 



11. If very light-absorbing 

 objects must be projected, use a 

 white background ( 285). 



1 2 . Use a hand-feed arc lamp 

 for opaque projection ( 289, 

 290). 



13. Make sure that the wir- 

 ing is adapted to the heavy 

 currents needed for opaque pro- 

 jection ( 290). 



14. Use carbons of the proper 

 size for the current drawn 



( 290, 7S3a). 



1 5 . Make the images erect by 

 placing the object up-side down 

 for the vertical position, or with 

 the upper edge away from the 

 screen for the horizontally 

 placed objects ( 293-294). 



1 6. Use a mirror or prism to 

 avoid a mirror image on a ver- 

 tical, opaque screen ( 293- 

 295)- 



9. Do not light solid objects 

 so that there will be deep 

 shadows. Use two radiants, or 

 mirrors, or arrange so that the 

 light strikes the object directly, 

 not obliquely. 



10. Do not select badly re- 

 flecting objects for opaque pro- 

 jection. 



1 1 . Do not use a black back- 

 ground on which to place dark 

 objects. 



12. Do not use an automatic 

 right-angle carbon arc lamp for 

 the heavy currents needed for 

 opaque projection. 



13. Do not nm any risks by 

 using the heavy currents on 

 wiring not adapted to it. 



14. Do not use small carbons 

 for big currents. 



15. Do not get the images 

 wrong side up on the screen. 



1 6. And do not expect too 

 much in opaque projection. 

 Know the principles involved; 

 study fig. 90-91. 



