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866 HANDBOOK OF PROJECTION FOR
NOTICE. — If it is desired to move the lamp toward you it is only necessary to turn knob F to the right, but if you wish to move the lamp from you you must turn knob F to the left, and at the same time shove the lamp holder in. This is because the threads on threaded collar U, Fig. 360, engage with threads in part S, Fig. 360, while collar U merely butts up against the end of lug Y, hence turning knob F to the right will pull the lamp back, turning it to the left will have no effect, unless you shove in on the holder at the same time. Examine the parts and you will see how it works. Be sure and tighten thumbscrew G, Fig. 360, when you are through.
FOCAL DISTANCES.— Examining the optical diagram, Fig. 362, you will see what the focal distances must be. They are the distances recommended by the manufacturer of the apparatus, who certainly should know what is best. You doubtless will be inclined to question the recommendation of nine inches from front surface of condenser to aperture when a small diameter projection lens and eleven inches when a large diameter projection lens is used, since this is exactly opposite to the usual condition recommended, but the figures are correct nevertheless. The apparent contradiction in distances is due to the fact that the longer distance with the larger lens is necessary in order to overcome chromatic aberration.
CAUTION.— DONT guess at distances. Use a ruler and get them exactly right. You are not working with an arc lamp now, and cannot make surplus light available to offset the waste attendant on "guess work," therefore stop guessing and do your work right if you want satisfactory results.
LOCATING FILAMENT.— The lamp filament must be 3^ inches from the face of the collector lens, as per Fig. 362. By means of knob B, Fig. 361, move the lamp backward or forward until, using a ruler, you have the filament exactly that distance from the face of the lens. When you get the filament the right distance, close the lamphouse door and move the filament locater in the lamphouse door backward or forward until you can see the lamp filament through it, whereupon tighten the holding screws of the locator. This device is to enable you to place the filaments of new lamps you install the correct distance from the lens. Once set, as above, when you install a new lamp you have but to move it backward or forward until the filament can be seen through the locator tube, and it is exactly the right distance