The Optical Magic Lantern Journal (February 1894)

30 The Optical Magic Lantern Journal and Photographic Enlarger. the objective or front lens of the lantern. If a Kodak or other flexible negative be used it should be sandwiched between two pieces of glass tightly bound together with a gummed strip in the ordinary way, for unless this precaution be taken the heat of the lantern will cause the film to curl, rendering focussing difficult, besides sometimes giving rise to slight movement during the actual exposure and precluding the possibility of obtaining a sharp picture. Having the negative placed in position and the lamp lighted, the next proceeding is to ascertain whether the focussing arrangement will permit the lens to be removed far enough from the negative to give only a moderate degree of enlargement. It will probably be found that it is impossible to obtain a smaller disc than 20 to 24 inches and it is therefore necessary to provide a tin lengthening tube, to be inserted between the two tin or brass draw tubes that carry the lens flange ; one half of this extra tube must be made of a double thickness of tin so that it will go inside one of the original draw tubes and outside the other. Itshould be nicely fitted so as to allow of free movemcnt, but without shake. If the equivalent focus of the lens be unknown it should be determined by the method described by Mr. Goodwin Norton in the last number of this Journal, as this will be found to be a great help in deciding what length to have the supplementary tube, and will also save time in arranging the relative distances of the negative, the lens, and the board supporting the sensitive paper or plate. So far we have presumed that the objective of. the lantern is corrected for photography, that is to say, if the image appear sharply upon the card or paper used as a focussing screen that the resulting enlargement will appear equally sharp. Now although most lanterns are provided with portrait combinations as objectives, many of the higher class instruments have specially constructed lenses which are designed for the purpose or projection only, and which are not, from the photographers’ point of view, properly achromatised. Vhe photographer can, however, always resort to the lens with which he took the original negative, provided that the same be of a moderately short focus, and it will in many respects be found the best possible tool for the purpose, for it will tend to remedy any distortion which it may have caused in the negative, this being particularly true when a single landscape lens is in question. Any turner will make a disc of hard wood to serve as an adapter to screw the photographic lens into the lantern flange. For experimental purposes a bung with a carefully centred aperture in it will answer well ; although a wooden or metal adapter should be provided for regular work. The operator should, before having the lengthening tube made, decide what lens he intends to employ, as upon this the length of such tube mainly depends. A good rule is to make the greatest available distance between the optical centre of the lens and the surface of the negative double the equivalent focus of the former, This will permit of the projected picture being made of the same dimensions as the original negative or as much larger as may be desired. For great amplification the middle tube may be dispensed with and the original draw tube used alone. he last point to be noted is a very important one; it is the cutting off of the stray light which finds its way out of about fifty apertures, large and small, in the average lantern. No ray of this must reach the sensitive surface and yet the ventilation of the lantern must not be interfered with. This appears to be rather a ‘large order” but is in reality a very simple matter, as it can be effected by enclosing the lanternin a three-leaf screen or clotheshorse covered with dark curtains, a small opening being made for the lens to peep through, while a piece of blackened tin or ferrotype plate, 8 or 10 inches square, fixed upon the usual hood of the chimney will effectually prevent any rays from reaching the ceiling. If it be desired to construct a special screén, a hint may well. be borrowed from the unknown architect. of the earliest dramatic edifice—the Punch and Judy Show. ——:0: —— The Optical System of a Lantern. By A. M. Hrippon. In bygone days, when the optical lantern was but a toy, an accurate knowledge of the why and wherefore of its various parts was superfluous; but nowadays, when it has bcen elaborated into a delicate piece of highly scientific apparatus, demanding a considerable amount of technical knowledge for its proper manipulation and management, and when perfection of results is only to be obtained by a careful consideration of, and attention to, every detail, the theory of its action can no longer be utterly disregarded. In the earlier forms the lantern and its parts were nore or less incapable of adjustment, and the results obtained from it depended entirely upon the skill and care