Richardson's handbook of projection (1927)

942 HANDBOOK OF PROJECTION FOR Be sure and tighten thumbscrew G, Fig. 366, when you are through. FOCAL DISTANCES. — Examining the optical diagram, Fig. 359, 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. CAUTION.— DON'T 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. A Figure 367 B LOCATING FILAMENT.— The lamp filament must be 1-9/16 to 1-7/8 inches from the face of the collector lens, as per Fig. 359. By means of knob B, Fig. 366, 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 locater. 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 locater tube, and it is exactly the right distance from the lens. It is very important that this distance be precisely right. FOCUSING THE MIRROR IMAGE.— It is now time to bring the mirror image into play. It will be noticed that by swinging knob C, Fig. 366, to right and left that another image will appear on the cooling plate slightly fainter than that of the lamp image. This is called the mirror image, and should be centered first, that is centered up and down by means