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28 SCIOPTIOON MANUAL. Bafer. A lantern with a calcium light of either kind is commonly called a Stereopticon. The Magnesium light is excellent in photography, but it gives off troublesome fumes, and for a continued lantern exhibition is too liable, even with well-regulated clockwork, to leave the lookers-on in sudden darkness. The Electric light is intense and concentrated, but it requires too much apparatus to be available. The above so-called chemical lights, are, if well man- aged, much brighter than flame, even at its best. The Sciopticon has a double flame, which is not only very bright, but gives much more distinctness to the image, by its standing edgewise to the condenser instead of broadside, as a single flame must, to prevent its casting a dark shadow on the disk. No lantern of any sort can compare with the Sciopticon in point of convenience. We may conclude that the Hydro-Oxy-Calcium light is best for exhibitions on a large scale, and as for the rest, the Sciopticon is desirable as combining efficiency and great convenience. THE SIZE PROPER FOR THE ILLUMINATED DISK. The image enlarges in area, and diminishes propor- tionately in brightness, as the distance of the lantern from the screen increases. A disk of six or seven feet is about right for figures, statuary, <fcc., to give bright- ness and not an unnatural size; while landscapes, &c., appear better on a disk of eight or ten feet, or more. With an objective of about four inches back focus, as is most used on the Sciopticon, a distance of about sixteen feet from the screen gives a disk of about eight feet. The arrangement can be varied to suit circumstances.