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MOTION PICTURE PROJECTION 183
dition, we would make a selection from 5%" lenses in stock of those the nearest to 5.60" focus but longer rather than shorter. . Of course there is a possibility in every case that an error in measuring the distance will be a disturbing factor and some allowance should be made by the customer for some difference between the size of the picture and screen which is unavoidable and easily painted out.
Lenses are matched by selection as the focal length cannot be modified after they are finished. In manufacturing they deviate to some extent from the focal length prescribed by the optical formula running both under and over for which reason they are not necessarily the exact focal length engraved upon the mounts. For example, a 4" lens may vary within the quarter inch from 3.95" to 4.20", it being our practice to mark the mounts within 5/100" under to 20/100" over of the actual focal length and it will be perceived that two lenses marked with the same focal length may at the most have a difference of %" and matching for pictures of the same size necessitates that both lenses shall be exactly the same focal length. This being the case the lenses must be matched when they leave our factory unless a lens to be duplicated is sent to us so we can measure it or if it was purchased from us we will have a record of its focal length which we can locate if given the order number or date of invoice. The exact focus in hundredths of an inch is shown by our invoices in parenthesis, for example, (4.36), and purchasers should make a note of this to faciliate placing repeat orders for duplicates when they wish to match a lens or replace one which has been damaged.