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grains, green, red, masking, mosaic, light and screen

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COLOUR Sciwax PLATES AND FILMS 885. The Autochrome Plate a and Films. The microscopic particles of the mosaic' which forms the tricolour screen of the Autochrome are grains of potato fined by levigation, so that only those grains are retained which are between 8 and 20p, (thousandths of a milli metre) in diameter, with a marked predominance of grains of 12-16p diameter. These grains are divided into three parts, dyed respectively blue-violet, green, and red in concentrated solutions of suitable basic dyes. The colour must be very intense to ensure efficient absorp tion of the light by so thin a coating. After drying, these grains are intimately mixed in such proportions that the resultant colour is not far removed from a neutral grey.

Glass plates of practically constant thickness Vith in.) or films are uniformly covered with a tacky coating on which the mixture of coloured grains is dusted. The grains can therefore form only a single The spaces between them are filled in with very finely-powdered wood charcoal. The coated plate is then subjected to pressure to squeeze the starch grains into con tact. The pressure used is very considerable— over 30 tons per sq. in. -which is near the crushing limit of glass. The pressure is applied in continuous narrow bands (about 1/ ibth wide) After this species of rolling, the mosaic screen is protected by a thin coating of waterproof alcohol varnish, which, after drying, must have the same mean refractive index as the starch. The resins used must be of high melting point in order to avoid too rapid destruction in case of heating, and they must be without any action on the sensitive emulsion which is afterwards applied. The thickness of the three-colour mosaic and of the coat of protective varnish is about 15p.

A thin layer (about 4p) of a fine-grain pan chromatic emulsion is then coated on the varnish. This emulsion is proportionately richer in silver bromide than ordinary emulsions. The grains of the developed image have an average diameter less than and the clumps of grains are rarely more than 2/1 in diameter.

The coating of the emulsion on the surface of the mosaic screen is a very difficult operation. The emulsion film is easily detached by simple pulling when dry, or if the gelatine is much swollen by prolonged immersion in very alkaline or warm solutions. Very great care is therefore

necessary in manipulation. Finally, it must be expected that plates or films which have under gone so many processes should show some small defect from time to time.

The number of coloured grains is between 3,750,000 and 4,370,00o per sq. in., and as the green grains are less transparent than those of other colours' they are always more numerous. 2 In spite of the precautions taken to mix inti mately the threc kinds of grain, it is not possible to avoid clumps of several grains of one colour. These clumps are sometimes visible when an Autochrome plate is examined under a moderate magnification (iii the stereoscope or in the lantern, etc.), whereas the individual grains are only visible under the The average transparency of the completed mosaic, measured by white light, is about TO per cent under the most favourable circumstances.' 886. Principles of Autochrome Colour Photo graphy. Time mosaic screen, neutral grey when seen by transmitted light, appears white when illuminated by a sufficiently strong light, and if surrounded by a wide opaque border. If, on a portion of the plate, a deposit of silver of uniform density is superposed on all the grains of the screen, there will appear, under the above conditions, and by contrast with the " white " of the naked screen, the sensation of a neutral grey—darker as the silver deposit is denser. The complete masking of the blue grains, for example, in a given part of the mosaic gives the illusion of a continuous yellow tint as a result of the addition of the light transmitted by the green and red grains, while the complete masking of the blue and green grains will yield a pure red. The complete masking of the blue grains and the partial masking of the red grains will give a yellowish-green. The partial masking of the green and red grains will produce a sky blue by mixture of the green and red lights, transmitted at reduced intensity and of blue light, transmitted at full intensity. Finally, the complete masking of the blue and red grains and the partial masking of the green grains will give a dark green.

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