Ch. IX]
COMBINED PROJECTION
295
b 1 Block with four guiding cleats of wood. The screw holes in the inside
cleat at the left and the outside one at the right are made large, and washers
are used on the screws. This is to make accurate centering possible.
b 2 Block showing two guiding cleats between the tracks. Only one
cleat has large screw holes for centering.
. b 3 The third carrying block with the guide cleats on the outside of the
■ track rods. Only one has large screw holes for centering.
b 4 The fourth carrying block with guide cleats at only one end, and with
centering holes in one cleat.
At the left Sectional views of the carrying blocks.
In b i is shown how to make a table for carrying apparatus along the optical
bench, and at t s, the method of screwing the track tubes to the baseboard.
In b 4 is shown how to attach a shield with an opening for lantem slides (0).
two ways: (1) The arc lamp can be put closer to the condenser,
thus making the beam between the elements diverging instead of
parallel (fig. 1), or (2) a condenser lens of longer focus can be used
for the lantern-slide projection. In much of the modem projection
apparatus the condenser lenses are easily changed (sec fig. 166).
Fig. 160. Universal Level.
(Cut loaned by the L. S. Starrett Co.).
A level like this which serves for vertical and horizontal leveling is very
convenient and essential for projection work.
The second method of combined projection is to have two com¬
plete lanterns side by side, one for micro-projection and one for
lantern-slide work. In this case there should be a double-pole,
double-throw switch; then one can turn either lantern off or on at
will (fig. 162, 164).
Finally, in much of the modem apparatus special provision is
made for combined projection (see fig. 164-176).