CH. IX] 



COMBINED PROJECTION 



295 



b i 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 j 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 / 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 lantern slides (0). 



two ways: (i) The arc lamp can be put closer to the condenser, 

 thus making the beam between the elements diverging instead of 

 parallel (fig. i), or (2) a condenser lens of longer focus can be used 

 for the lantern-slide projection. In much of the modern projection 

 apparatus the condenser lenses are easily changed (see fig. 166). 



FIG. 1 60. UNIVERSAL LEVEL. 

 (Cut loaned by the L. S. Starred 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 modern apparatus special provision is 

 made for combined projection (see fig. 164-176). 



