The ether-oxygen or ethoxo limelight is similar to the last, except that vapor of sulphuric ether is used in place of the ordinary hydrogen gas. It gives a very beautiful light, but has several drawbacks, dan ger of explosion being one, so that it is but very little used.
The Electric Light.— For some time it has been apparent that the most perfect illuminant for the optical lantern would be the electric arc not only on account of its intense whiteness but principally for the reason that the light is confined more nearly to the theoretical point.
The above cut shows one of the better forms of electrical lanterns manufactured by J. B. Colt & Co. In this form of lantern, the bellows, front and objective supports, can be readily removed, admitting of the use of the microscope or other scientific attachment, and various combinations of condensing lenses may be used as the case may require. The hood or light guard between the light and condensers is of unique and original design, consisting of two cylindrical parts, one of which surrounds the carbons, and the other is secured to the back of the lantern. As shown in the cut, the smaller section of the hood telescopes into the larger section, which is supported on the lantern without coming in contact with it thereby obviating the direct conduction of heat from the lamp to the condensing lenses, and as the outside cylinder is of considerably larger diameter than the inner one, perfect ventila tion is secured and also perfect freedom for the adjustment of the light in all directions in rela tion to the lenses. When exhibitors are located on a low tension continuous current, the auto matic lamp has advantages over any other form, but if this is not the case and a high tension current or an alternating current must be used, a lamp of different construction is necessary. The one that this firm have constructed for this purpose is not automatic in its control of the carbons, but is provided with a simple mechanism whereby the carbons are moved toward each other every few moments as they are consumed. This lamp is exceedingly simple in its operation, and the adjustment of the carbons is no more difficult than the occasional turning of the lime in the well known oxy-hydrogen apparatus.
The object of the condenser is to collect as much of the light as possible, and pass it uniformly through the transparency. There are two principal forms—one consists of a pair of plano-convex lenses, mounted, with the curved surfaces nearly touching, and the other consists of a double convex lens associated with a meniscus, the concave side of the latter being next the radiant point.
The Lens, or objective, is required to receive the rays of light passing from the condenser through the picture, and to magnify and project them on to the screen. It should have a flat field and be free from distortion. Further, it should be achromatic.
How to Use the Optical using the optical lantern the following hints will be found useful.* Wipe the front and condensing lenses with a piece of soft silk free from every particle of dust. Thoroughly dry new wick before putting it into the lamp ; let the wick be saturated with oil before lighting. The best paraffin oil gives the brightest light. Diener's A i safety oil, and Strange's Ai crystal oil, also the Vaseline Company's Luxor, are specially recom mended. The best quality wick is indispensable to brilliant illumination. Cheap wicks and cheap oils are false economies, and answerable for very many failures and disappointments when exhibiting the lantern. A large and strong pair of scissors should be used to trim the wicks, or preferably the "Optimus" patent wick trimmer, as a perfectly straight and even edge to the cotton is necessary to the avoidance of an uneven flame and smoke. Cut off the protruding cor ners to allow the flame to draw evenly upwards. Light the lamp with wax vestas or tapers. The heads of matches or charred paper are liable to fall into the air passages between the wick tubes, so impeding complete combustion. When lighting the lamp, do not turn the wicks high immediately, but raise them little by little at intervals of a few minutes. This system, besides improving the flame, gradually warms the condensing and front lenses ; if suddenly heated they are likely to crack. Cold air being allowed to blow on a heated condenser will also certainly cause unequal contraction, i.e., a cracked glass. The oil reservoir and other exterior parts of the lamp should be wiped perfectly dry, otherwise the oil about them will vaporize and fill the apart ment with a most unpleasant odor. The wick should be trimmed prior to each exhibition, and when not burning should be turned one-eighth of an inch below the top of wick tubes, so that they may be permeated with oil. A plentiful supply of pure air is an imperative necessity to perfect combustion or perfect illumination. In crowded rooms, opening a window or door will greatly improve the light of the lamp ; insufficiency of air ensures a bad light and an unsatis factory exhibition. Like ourselves, the lamp gives its best results when it enjoys pure air, clean surroundings, and pure food (oil).