CH. XII] DO AND DO NOT WITH ROOMS AND SCREENS 



473 



9. If possible place the pro- 

 jection apparatus at the back of 

 the room (612). 



10. The axial ray of the image 

 beam should strike the screen at 

 right angles (614). 



1 1 . Incline the screen if neces- 

 sary (614). 



12. Light the black-board by 

 lights behind a curved, metal 

 shield (fig. 240). 



13. If a translucent screen is 

 used the objects must be put 

 into the apparatus with the pic- 

 ture facing the objective (516). 



14. Use a good screen ( 621). 



15. If you use a mirror or 

 metalliic screen remember that 

 it does not reflect equally 

 throughout the 180 degrees 

 ( 629). 



1 6. Make the screen image 

 large enough so that the most 

 distant spectator can see the 

 details (633, 639). 



9. Do not place the projec- 

 tion apparatus in the middle of 

 the room if it can be avoided. 



10. Do not let the axial ray 

 strike the screen obliquely. 



1 1 . Do not incline the screen 

 unless necessary. 



12. Do not try to use the 

 blackboard without lighting it 

 by hidden lights. 



13. Do not forget the rules 

 for erect images when using a 

 translucent screen. 



14. Do not use a dirty screen. 

 Wash it or give it a fresh coat of 

 white paint. 



15. Do not use a mirror or 

 metal faced screen in a square 

 room ; such screens are not good 

 for the fine details necessary in 

 micro-projection ( 360). 



1 6. Do not have the screen 

 image too small nor too large. 



