The sciopticon manual, explaining lantern projection in general, and the sciopticon apparatus in paricular (1877)

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SCIOPTICON MANUAL. 27 the incident rays a b c. Some advocate a larger reflec- tor, a parabolic reflector, or reflectors at the sides; but unless the reflected light coincides with a b c, or nearly so, it is not transmitted by the objective lens. "With proper adjustments, the light J, with that from the reflector, is focused at the objective tube, of a com- parative size proportionate to its relative distance from the condenser; and the picture p is focused upon the screen at i of a comparative size proportionate to its relative distance from the objective. As represented in Fig. 14, the smallest diameter of the cone of light in the objective A B would be twice that of the point of light I, and the height of the image i would be twice that of the picture^. To project a picture to a great distance without too much enlargement, the objective must be of low power and carried forward; and the light should be from a point (as in the calcium light) to avoid loss, and should bo carefully adjusted to secure even illumination. The longitudinal ridge of light E (Fig. 15), with a medium objective gives uniformly good results without perplexing experimental adjustments. VARIOUS MODES OF LANTERN ILLUMINATION. The Hydro-Oxy-Calcium light, or lime made incan- descent by a jet of hydrogen and oxygen in flame upon it, is the most brilliant available light. Its concentrated form adds greatly to its value for the lantern. The Oxy- Calcium light produced by a jet of oxygen on to lime, through an alcohol flame, is much inferior to what is produced by the mixed gases. It is much used, because ono gas is easier to manage than two, and it is counted