The book of lantern ; being a practical guide to the working of the optical (1888)

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5.6 THE BOOK OF THE LANTERN. milled heads. The arrangement is by no means a good one, but it is one of those simple things which have been unheeded by the many, and manufacturers generally have adopted it without perhaps thinking how inconvenient it is in practice. In the first place, the jet is apt to slip either downwards bodily, or to one side or the other by a careless touch of the operator's hand. In either case, such a shift- ing of the light out of the optical axis causes the disc to be darkened until the jet be readjusted, which cannot be easily done without opening the door of the lantern and letting out a flood of light in the darkened room. There is, besides, much trouble in getting the jet central, which could easily be avoided by a more rational arrangement. Mr. Pumphrey, of Birmingham, has devised for the purpose a horizontal and vertical rack motion, very like the same movement which is attached to the mechanical stage of a microscope, but it is somewhat expensive, and FIG. 24. adds extra weight to the lantern. There is no doubt, at the same time, of its effectiveness and conveni-