The history of three-color photography (1925)

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542 History of Three-Color Photography spirit lamp, and the shutter immediately opened. When the magnesium is exhausted the plate is removed and developed as usual. The time of exposure, that is the length of ribbon is easily determined by a preliminary test. It should be as exact as possible to secure a good reproduction. Exposure must, of course, be conducted without any other light than the magnesium. Copper or brass wire should not be used, and the diameter of 0.3 mm. should be adhered to as closely as possible, being suitable for magnesium ribbon 3 mm. wide and 0.1 mm. thick. To make the spiral it will be found convenient to wind the wire on a round object of 3 to 4 mm. in diameter, in such a way that the rings are juxtaposed. The support is then withdrawn and the spiral slightly extended. When the magnesium has been inserted, it is again stretched until of the desired length, that is, until the rings of the spiral are 1 centimeter apart. Instead of the wooden box a camera of the same size may be used, the lens of which is replaced by the special yellow filter, and provided the plate holders are sufficiently deep to hold two plates. Fig. 138. Fig. 139. Von Hiibl5 also dealt with the subject from a simple geometrical standpoint, which considerably facilitates the comprehension of the basis of printing. When an Autochrome plate is printed on to printing-out paper, so as to obtain a print in monochrome, the results are weak because practically the only light that affects the silver salts is that passing through the blue particles, therefore, the whole print is broken up into minute areas of white plus black. So that even the deepest shadows are represented by one-third black and two-thirds white, and the same difficulty is met with in reproducing on another screen-plate. The fact that such exposures can only be made by placing the film side of the original towards the glass side of the plate on which the copy is to be made, and is, therefore, about 2 mm. distant from the sensitive surface of the latter, is not of much account, as it is easy to obtain sharp copies. A camera can be tilted towards the sky, which must be covered by white clouds, and the lens set at infinity thus securing parallel light; but the difficulty of color rendering still comes into play. For if we suppose that the red consists in the original of 300 red and 600 black grains, then in reproducing only one-third of the red grains will fall on the red grains in the second film, and the result will be 100 red