The Optical Magic Lantern Journal (August 1893)

112 The Optical Magic Lantern Journal and Photographic Enlarger. brought opposite the focal plane. Subsequent turning of the key brings other figures, up to 12, opposite the aperture, each figure denoting that sufficient film has been unrolled for another exposure. After the figure 12 has been reached the tilm ceases, but there is a sufficiency of black paper left to be rolled on the reel to protect the film from light after the cartridge has been taken out. The paper having all been wound on the reel, the cainera may again be opened, the exposed cartridge removed, and a new one slipped into place and adjusted as before in daylight. This instrument is provided with a finder, ever-set time or instantaneous shutter, and single achromatic lens. It sellsat an extremely moderate price. : -——10: —— Normal Capacity of Cylinders. By Horacz Hinvon (Brieuron). One or two correspondents when speaking of the normal capacity of gas cylinders, appear to be somewhat mystified. The following table which explains itself, may aid them in their difficulty :— Contents of cylinders charged at 120 atmospheres. 120 cubic feet. Normal capacity of cylinder. 1 cubic foot. 100 43 2 ” 80 ”) 2 ” 60 » 4 ” 50 me vs ” 40 ” 7 . : ” 25 ” oz @ », 20 ” ¢ g ” 12 ” qo EI oy 10 ” tus ” 8 7" ts ‘oy 5 ” ors » :0.—— Oils and Lamps for the Lantern.—No. III. By W. H. Harrison, THE nature of the wicks used having been shown to have considerable influence upon the light-yielding character of petroleum lamps, the next point deserving attention is the nature of the oils. Keeping the outside of the lamp clean and free from oil is well known to have much influence in either preventing smell altogether, or in reducing its amount; but there are some oils in the market that even Mr. C. H. Bothamley, with all nis training io chemistry, was unable to burn in the lantern without an objectionable amount of smell, and the specimens in question were found to come from the Black Sea. The supply, or rather the transit, of Black Sea oils is increasing, and has long since pretty well broken up the consumption of American petroleum—which is its great competitor—in the regions immediately surrounding the Mediterranean. The _ hydrocarbons composing Black Sea petroleums are not the same chemically as those composing American petroleums, These facts should therefore be borne in mind by the users of lanterns, and as the quantity of oil burnt in lanterns is so small as to make a slight increase in expense no object, dealers might do well to | keep on sale for thei Scotch paraffin oil as supplied to lighthouses, which is not objectionable as to smell when properly used; it has flashing points of 130° I. to 140° F., and much higher for lightships ; this oil as used in lighthouses is not allowed anywhere in the lamps and reservoirs to reach a higher temperature than 86° F, A curious fact about some mineral oils is, that at high temperatures they have been known to decrease in illuminating power. A statement was once made before the Glasgow Section of the Society of Chemical Industry, that Dr. Stevenson Macadam knew of one instance in a lighthouse in which an extra series of reflectors, by increasing the temperature so diminished the light that they had to be removed. A reflector does harm in a magic lantern when the centre of the circle of which circle or globe it forms a part, does not coincide with the centre of the flame of the ring burner ; but if placed properly and worked to the proper curve it is beneficial with argand flames. The possibility of its diminishing the light of the flame by increasing the heat thereof should be borne in mind, altbough it is highly improbable that any such case will practically occur in connection with the lamps used in magic lanterns. The demand for lanterns of small bulk is all right when the lime light is used, but is all wrong when applied to oil lanterns. The latter should be large, and have particularly large air-channels, especially below, to help to keep the oil reservoir cool, and the roof of the lantern should have such a shape and so little polish as to reduce its reflecting power to the lowest ; if it were a concave mirror let us say for the sake of argument, the reflected rays would make the oil reservoir below exceedingly hot. A useful thing for lanternists would be a series of experiments with different lanterns in the market, to ascertaln what temperature the