The book of lantern ; being a practical guide to the working of the optical (1888)

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00 THE BOOK OF THE LANTERN. doubtedly the best thing of all is a simple white-washed wall. Why should this be so ? Let me endeavour to explain the matter in a few words. Suppose that we go behind an ordinary linen or cotton sheet, while lantern pictures are being thrown in front and upon it. We not only see the picture distinctly through the sheet, but there is enough light round about us to enable small print to be read with ease. At least such is the case with the lime-light, and in a minor degree with the oil lantern. Now all this light means so much deducted from the light available from the lantern, and which, therefore, is completely lost to the spectators in front. From an opaque wall, on the other hand, the light is nearly all reflected to the spectators' eyes; and although no doubt some must be absorbed, we know that none is actually transmitted through the screen, and utterly wasted, as in the case of a semi-opaque sheet. We might compare the two cases, to a vessel of water with a porous bottom, which will, of course, allow a large portion of the liquid to dribble through and be lost; as against a vessel with a solid bottom which will hold water without any wasteful transmission. But a white-washed wall is not often met with in a lecture-hall, and the best substitute is a canvas sheet rendered opaque with white-wash. Such a sheet is portable up to a certain size ; and if intended for use as a fixture in any one hall, can be made up to any size, within reasonable limits. Witness the scenes on rollers at our large theatres and opera houses, the basis of which is simply whitened canvas. Such a plan was adopted at the late Polytechnic Institution, where the screen measured no less than 26 feet across.