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24 OPTICAL PROJECTION by such loss from reflection at the edges of the first lens, owing to the high angle of incidence, that the edge of the disc would be less illuminated than the centre. Practically the angular pencil is thus limited to somewhere from 65° to 70° in lanterns for exhibiting slides. 18. Triple Condensers.—Condensers of three and even four lenses have been used, especially in America. Theoretically, they admit of more perfect correction for aberrations, and a larger angle of light; but this is complicated by the number of extra reflecting surfaces. I have perfectly satisfied myself, that for exhibition lanterns they afford no gain whatever. For large condensers, as five inches and over, and where the light is to be condensed upon a small surface, as for the projection microscope, they are of advantage, and allow us to use a pencil of 90°. Such a condenser will be described in connection with the instrument just named. 14, The Slide-stage.—Little need be said here about this. For a single lantern only used to exhibit simple slides and diagrams, it does not matter much how it is constructed. When, however, experiments may have to be made in the stage—as in a chemical tank, and for some ' effects' with slides—as the ascent of a balloon, it is important that the stage be open at the top, with the exception of the pillars needful to carry the objective mount. Also, for anything like general work, it is important that while the spring pressure-plate allows the slide to be inserted easily, for which purpose the edges should be carefully turned back to a smooth curve, the slide should be held firmly when in place. If it be not so, the working of any mechanism, as the handle of a chromatrope, may move the slide about in a very unpleasant manner. Both these requirements should be attended to, even in a cheap lantern of japanned tin. 15. The Objective.—This is the lens directly employed in the projection. A simple lens is never now used except in toy lanterns, and for physical experiments, the latter for reasons