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APPENDIX 439 so that one image is polarised in one plane and the other in the opposite (see Chap. XXII.). These images are superimposed on the screen, but cannot be made to coincide more than approxi- mately, the result to unaided eyes being a tangled confusion of both. But when the screen is viewed through a pair of eye-pieces arranged as spectacles, also containing ' piles ' of thin glass, only one image can be seen by one eye, and the other by the other eye, and any point of the two can be brought into perfect coincidence as in the stereoscope. Hence we have perfect stereoscopic vision, or, by reversing the eye-pieces, pseudoscopic effect is produced. If the screen be viewed while the observer walks across the room, the effect is very startling. Plain silver-leaf screens give a very bright image viewed by an audience directly in front, but the brilliance becomes very much impaired as soon as the observer moves a little to the side of the room, owing to the large amount of direct reflection, the silver surface acting almost like a mirror, at any rate so far as illumina- tion is concerned. To avoid this effect, my father suggested striating the surface with fine vertical ridges, and the plan proving perfectly successful, the screens are now made in this way. The introduction of * anaglyphs,' or pictures where the stereo- scopic effect is produced by printing the two images in different colours and viewing them through spectacles with the glasses correspondingly coloured, naturally suggested producing the same effect on the screen, and Mr. T. E. Freshwater introduced this method with considerable success. All that is necessary is to bind up each slide of a pair with coloured cover-glasses, say red and green respectively, and view the confused superimposed pictures thus obtained through the coloured spectacles. The method has the great advantage of cheapness and simplicity, and also of being independent of any special screen. Either of these methods necessitates a very strong light; for the colour method the electric arc, owing to its deficiency in red rays, is not so suitable as lime-light, but by choosing colours adapted for it, this defect could probably be overcome. Dr. Dupre has designed a method by which a revolving shutter closes the nozzle of each lantern alternately, and a synchronous revolving shutter revolves before the observer's eye; but such a method is necessarily too complicated and expensive to ever come into general use. June 1906.