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. 



