The history of three-color photography (1925)

532 History of Three-Color Photography num powder) should be dusted over it, whilst in a flat position, and then allowed to drv. On standing up vertically excess of powder will fall off. Zeiss, of Jena, introduced commercially screens of this type with a rippled surface, which enabled spectators at a wider angle to see the pictures. Lehmann gave the following table of the relative maximum brightness H, and the useful angle of diffusion W, that is the angle over which the metallic surface appears equally bright : H W Ordinary white paper 1.0 — Matt aluminum, Zeiss 13.8 48 Canvas aluminum, Zeiss 7.8 61 Rippled aluminum, Zeiss 3.4 84 Aluminum in celluloid 3.4 71 Aluminum in rubber 2.9 56 Coarse ground glass, silvered on the matt side. .1.6 96 The "rippled" screen was an aluminum screen to the surface of which a fine rib or ripple had been mechanically imparted. Von Hubl110 pointed out that if the pictures were projected in a square room, it follows from measurements that have been made, that one portion of the audience will see the projection in its full strength. Others will see the picture of only five times the brilliancy, and others still further removed from the center of the room will see it only twice the brightness, or even no brighter than in the ordinary way. While those far removed towards the sides, are at such disadvantage that they will see it of about half the brilliancy as projected in the usual way. It will thus be seen that only about 10 per cent of the audience can see the picture in its full brilliancy. While 50 per cent see it considerably better and the remainder not so well as in ordinary projection. In the case of the rippled screen, the conditions are more favorable; although the extreme brilliancy attained is only 3J/2 times as great as with a white screen; 20 per cent of the audience are able to view it at full strength. In this case also, the brilliancy falls off as a more sideways observation is taken, until the brightness is only that of the ordinary screen. Naturally the color of the light has considerable influence on the resultant impression, and details of the colored filters for projection will be found in the section dealing with filters. The early issues of the Autochrome plate were of a much more tender nature, as regards the film, than the later ones, and showed a great tendency to crack in the lantern. As a remedy for this von Hiibl111 recommended bathing the plate in a 5 per cent solution of glycerol and drying without washing. But as glycerol is a sensitizer for some of the dyes, and caused them to fade, he later112 suggested the addition of 5 per cent of cupric sulfate to the bath. The particular virtue of this addition is open to question, as the dyed elements are protected by an insulating varnish. H. E. Corke113