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LENSES AND MIRRORS 119
tiling of this sort should be done by all projection lens maker-. (82) Projectionists who have lenses in which the elements can be wrongly assembled will do well to
secure a bottle of waterproof draughtsman's ink, black <»r red in color, and as they disassemble the lenses for the hrst time, put a small spot of ink on the face of each lens just at its outer edge on the screen side. The spot should remain indefinitely. The ink or other mark must he near enough the lens edge to be hidden by the retaining ring.
(S3) Exercise care and patience in disassembling. The retaining parts have very line threads and are for the most part rather light. They may be bent out of shape quite easily. Do not attempt to force them by using a screwdriver point or other heavy instrument. I nthread with your lingers if it is at all possible to do so.
(84) When reassembling clamp each separate lens into its retaining mount snugly, but be very careful not to set the holding element up too highly. If you do, it will make it difficult to disassemble the lens again when
♦ necessary. It will be sufficient to set the ring up so that the lens does not rattle audibly when shaken. Screw the retaining mounts into the barrel as far as they will go. Failure to do this will disturb both the E. F. and the corrections of the lens.
Loss Through Absorption
(85) Glass used in modern projection lenses absorbs but a small percentage of the light as it passes through the body of the glass itself. Absorption of good crown glass is given as 1 to 1.5 percent per centimeter ( .3937+ of an inch) of distance covered by the light. Flint glass absorption is just a little higher. (86) Poor quality glass may absorb as much as five percent of the light per centimeter of thickness. Since condensing lenses of the ordinary piano convex 4.5-inch diameter type are thick, it is apparent that a condenser lens made of good grade glass is to be preferred to one of poor quality glass which absorbs light excessively.