The Optical Magic Lantern Journal (June 1892)

The Optioal Magio Lantern Journal and Photographio Enlarger. 65 which illuminates the slide, and that from the position of the slide itself. Mr. J. Traill Taylor, in his recent work, ‘ The Optics of Photography,” speaking of conjugate focus, says: “If the principal or solar focus of a lens be regarded as the unit of measure, an object situated in front of the lens at a distance from a certain point equivalent to a multiple of the said unit, will have its conjugate posterior focus at a distance from another certain point equal to a corresponding fraction of the same unit. Supposing we have a lens of 12in. solar focus, an object situated at a distance of 6ft. from a certain point in front of the lens, that is, at six times the unit of measure, the conjugate posterior focus will be at a distance of one-sixth of the same unit, that is, at 2in. distance from a corresponding point behind the lens. The point here spoken of before or behind the lens is the solar focus measured from the optical centre of the combination.” To ascertain the distance between lens and enlargement, multiply the focus of the lens by the times of enlargement required and add the focus. Example—enlargement to be five times; lens, rOin focus. Io X 5 = 50 + 10 = 6oin. For the other conjugate focus, divide the focus by the times of enlargement and add focus. 10 divided by 5 = 2 + 10 = I2in, 302 How to Renovate a Lantern. By C. Goopwin Nor Ton. No, 2,—JAPANNING. THE best lanterns are lined with russian iron, which does not readily tarnish, as it soon becomes covered with fine dust from the lime, or with a deposit from the fumes of paraffin, either of which is a good preventative of rust. Spots of rust, however, will sometimes appear; these may be removed by rubbing them with very fine emery powder and paraffin, afterwards merely wiping off the oil with a rag. But if the whole or greater part of the iron is rusty, as sometimes ’ happens, and the joints are rivetted, not soldered together, the process known as blazing off may be tried. The ironwork is well scoured with emery cloth to remove as much of the rust as possible ; and is then thickly smeared with any kitid of animal fat. The whole should now be heated sufficiently to set the grease in a blaze, and kept so until thoroughly consumed. This will leave a black deposit on the iron, which, while still hot, should be vigorously rubbed with a coarse cloth, a handful of shavings or paper. Thisprocess, if properly carried out, will produce on the ironwork a dull black polish, which will not smell when heated, require little cleaning, and not rust unless exposed to prolonged damp or some powerful oxidising agent. Out of doors is the best place to try blazing off, as it creates a great deal of smoke and smell. Wood shavings, paper, or straw, will be found suitable to make the fire with. Small articles can be satisfactorily blazed by holding them in the tongs over a kitchen fire. No greater heat should be employed than is necessary to consume the grease, or the deposit will be burnt up, instead of being left on the metal. Of course, nothing but iron can be treated in this manner. Tinplate is merely thin iron coated with tin, which would run off or form into lumps on the surface, and any article soldered would fall to pieces soon after the grease began to blaze. To re-japan the lining, dome, and rose chimney of a lantern is not a very difficult matter, provided the lanternist has access to an oven in which to bake them after the enamel has been applied. First clean off as much as possible of the old japan by scraping and washing in a strong solution of soda, then well rinse with clean water until all traces of the soda have disappeared, and thoroughly dry the articles in the sun or before the fire. Black japan for the purpose can be purchased at most oil-shops. It is applied with a soft brush, just sufficient being laid on to cover; the brush marks are erased by rubbing gently all over the surface with the tip of the finger. When dry it is placed in an oven heated to a little above the temperature of boiling water, not much higher, or the solder may run. The work should be left in the hot oven until the enamel is quite hard. The hotter the oven can be made within the limit mentioned, and the longer the japan is kept in it, the less likely will it be to chip off, or smell when heat is again applied. One coat may be enough for old work, but for new tin-plate two or three thin ones are best. Careshould betaken when applying thejapan not to put on too much, especially on the ornamental rose-top at the edges, or in the grooves where the trays slide. These grooves are often better not japanned at all, as they are never seen by anyone except the operator, and it is most annoying for the tray to jam and refuse to move, which sometimes happens when the lantern gets warm. If the lanternist has.a taste for the beautiful, he may like to decorate the top and dome of his lantern with gold or silver lines or figures, which can be