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438 OPTICAL PROJECTION Omni Focal Lenses.—Lantern lenses on the same principle aa the Tele-photo camera lenses of Messrs. Dallmeyer are now to be obtained, and such a lens gives very great adaptability in focal length. Messrs. Dallmeyer and Newton have conjointly placed on the market an omni-focal attachment which can be adapted to almost any good double achromatic lantern lens. A 6-inch lens fitted with such an attachment can be used as a 6-inch, 12-inch, or any longer focus required up to about 24-inch. The lantern simply has to be placed at the required distance from the screen, and the lens focussed. If the resulting picture is too large, the draw-tube adapter must be pulled further out, and the lens can then be focussed again in its new position, and a smaller picture will result. Unfortunately, up to the present, it is not possible to slightly lengthen the focus of a lens by means of such an addition. The omni-focal attachment, when it is screwed on, about doubles the original focus, i.e. a 6-inch lens becomes a 12-inch lens. Beyond that, almost any length of focus can be obtained. Of course, the attachment can always be taken off, and the original lens used by itself. lantern Stereoscope. —Desire has often been expressed that the effect of stereoscopic slides could be produced upon the screen, and various attempts have been made without success. Two stereoscopic pictures can, of course, be easily superimposed upon the screen, but for stereoscopic vision some arrangement must be made by which each image can be seen by one eye only, and this on so large a scale introduces considerable difficulty. It had once or twice been suggested that polarised light might solve the diffi- culty ; but nothing, so far as I know, had ever been done in that direction until Mr. John Anderson, of Birmingham, successfully worked out an apparatus, which was publicly exhibited at the Royal Society's summer conversazione in 1893. The really new and essential point in this invention is the screen, which is faced with thin silver leaf, varnished to prevent oxidation. This surface, besides its greater power of reflection, possesses the property of reflecting polarised light with its polarised properties, not depolarising it, as most surfaces do. The rest is easily understood. Each slide of a stereoscopic pair is shown from one nozzle of a biunial lantern (or a pair of single ones), and the image is transmitted through a ' pile ' of glass plates, arranged