The Optical Magic Lantern Journal (June 1898)

96 The Optical Magic Lantern Journal and Photographic Enlarger. — PoLaRiscorpe.—An instrument for demonstrating the phenomena of polarised lights. PowaRIseD Licnt.—Light differing in some of its pro| perties to ordinary light, behaving in a variety of ways with substances in agreement to the manner in which it falls upon them, and appearing as it were to take sides. Sec O.M.L.J., Vol. VII., pages 51 and 63. Po.yopricon.—Merely another name for the aphengescope. Pops.—What are called pops by lanternists are but small or miniature explosions. PorTABLE SCREEN STanps.—Those made to fold up into the smallest space as a convenience in travelling. Porassium.—The metallic base of potash. Press RuLEs.—As many readers may be induced from time to time to put their ideas and experiences in the form of an article for, say, the O.M.L.J., a hint or two may not be out of place. On the front outside page write your name, address, title of article, and number of words it contains, as follows, only one uader the other. ‘‘ The Lantern in Danger ’’—500 words—Robert Brown—Rose Cottage—Barnet. A penny manuscript book is about the handiest form to use, and only to be written on one side of the paper ; this must be religiously observed, otherwise the poor compositor would often be in a sorry plight, for be it remembered an article is sometimes distributed among many men, and shoulda sheet be cut into two or three pieces, the greatest confusion would prevail. Let your diction be as concise as possible. The MS. should be forwarded flat if possible, if that is not convenient, one fold down the back is next best, but never under any pretence is it to be rolled. If typewritten so much the better, if not, the caligraphy must be plain and distinct. All the sheets must be fastened together at the top left hand corner with a paper fastener, or sewn. PressuRE BoarpDs.— When rubber bags are used for gases it is necessary to place weights of some kind upon them to increase the pressure, and this is best done by using pressure boards which are made for the purpose ; they are hinged together to form a wedge-shaped box, battered outside with ledge-on cross-bar for weights to rest upon and keep them in place. The pressure board adds greatly to the durability of the bag. They are easily made, care being given to the sides of the board that comes in contact with the bag; these should be perfectly smooth. To keep the bags in their place | whilst between the boards, leather straps will verve best. Pressuge GAvuGE.—All lanternists should have in their kit a cylinder gauge. Nothing is more annoying than not to know the quantity of gas at disposal. These gauges Gre made in various sizes corresponding to the sizes of cylinders, and properly made show at a glance the number of cubic feet of gas the cylinder contains. A word of caution, don’t use the same gauge for the two gases, but keep one for cach. PrRoJEcTION.—Magnified objects, pictures, phenomena and so forth, when thrown upon the screen by means of natural or artificial light, in conjunction with lenses for the purpose, is known as projection. PyrotyLin.—Gun-cotton and other substances of explosive kind. Vegetable fibre is soaked in a solution of nitric and sulphuric acid, and after several washings allowed to dry. The process is simple, and if attention be given to detail, it may afterwards be stored for an indefinite period.—See Collodion. (To be continued.) Photographic Perspective with Lenses of Different Types. By T. PERKINS. is frequently asserted that certain lenses—those of the wide-angled family —produce pictures with exaggerated or distorted perspective. Let us examine the question thoroughly, and see how far there is ground for this charge. Any lens, whatever its character, will falsify perspective if the camera is tilted so that the plate or focusing screen is out of the vertical, or if, the lens being horizontal, the side swing is used. We will therefore assume that lens and camera are so arranged that the axis of the lens is horizontal, and the focusing screen perpendicular to the axis, and therefore absolutely vertical. We will, moreover, assume that the lens is rectilinear, for mn ? every photographer knows that a single lens gives rise to curvilinear distortion of all straight lines, except those which pass through the axis of the lens, the distortion being more and more pronounced the further the lines are removed from the axis. These assumptions being made, we maintain that there is absolutely no distortion or exaggeration of perspective, whatever be the character of the lens—rapid rectilinear, wide-angled rectilinear, or modern anastigmat. Let us plant the camera, which we take to be a whole plate one (83 by 64), ready to photograph some architectural object, for it is in such photographs that exaggerated perspective, if it exists, will be most easily seen; and let us procure one lens of each of the above types, all of the same focus, say 5 inches, and proceed to photograph the building (with the three lenses successively), using any stop we find needful to secure the definition we desire, for the stop has no influence on the perspective, and taking care not to move the camera when changing the lens. We will then procure a lens of any of the above types of 10 inch focus, and leaving the camera unmoved, saving that we must rack out the lens or back to focus the view, expose a fourth plate. We will now develop the four plates. The one exposed by means of the R.R. of 5 inch focus will probably have corners which have not been affected by light because the circle of illumination‘is not sufficient to cover the plate; the other three plates will be fully covered.