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).
11. 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).
16. Make the screen image
large enough so that the most
distant spectator can sec 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.
11. 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).
16. Do not have the screen
image too small nor too large.