The history of three-color photography (1925)

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Screen-Plate Practice 529 solution of the primary image, through too short action of the reversing solution; (b) to the reversing solution having been used too often, and thus having lost its solvent power; (c) according to the makers, to traces of undissolved silver remaining through small inequalities of the film thickness; (d) a precipitate of manganese dioxide from the second clearing solution; (e) to the developer not having been thoroughly washed out, thus reducing the intensifier. The remedies are (a) longer action of the reverser; (b) the use of freshly made reverser ; (c) more complete action of the reverser; (d) the application of a solution of oxalic acid, or sodium bisulfite, which dissolve the manganese compound. Peculiar patches are sometimes also caused by the back of the plates not being clean. Dirt here would naturally prevent access of light to the emulsion, which would not then be reduced to metallic silver, or not to so great an extent, by the first developer, and after reversal the second development would reduce the unaltered silver bromide. There is no remedy, or the very doubtful one of local reduction. Green Spots. — These difTer in character. Small spots are due (a) to minute holes in the insulating varnish, through which the solutions penetrate and dissolve out the dye, which thus spreads a little; (b) larger spots and lines are due to mechanical injury to the varnish through too great pressure of the springs of the plate holder; (c) to allowing the plate to remain wet too long after washing; this particularly shows at the edges. The remedies are (a) spot out with opaque color; (b) thicker cards and reduction of the pressure of the springs; (c) gently dab ofT moisture with a soft cloth; whirl the plate till dry, or put in a current of warm air. White Spots. — Generally due to air-bells clinging to the film during the second development. These must not be removed with the finger, as the film is so tender, that it is almost certain to be ruptured. A small pledget of absorbent cotton, well wetted, should be lightly passed over the spot. If the plate is placed in the reversing solution in white light, traces of the developer may reduce unexposed silver bromide, which would then dissolve ; or the light might cause slight printing-out of the silver bromide, when it would also be dissolved. Blue Spots. — These, it has been stated have been occasionally met with, and are ascribed to the alcohol penetrating the varnish and dissolving the blue dye. This, however, is doubtful. If the image appears blue generally, with practically little other color, it may be due to the omission of the compensating filter, or if there is a general blue tinge, the plate is underexposed, or the filter has faded. Stray white light would also naturally produce the same effect, because it would penetrate the blue screen elements and cause more action thereunder than through the red and green, therefore the former would be more transparent in the positive. Von Hiibl104 stated that the mosaic screen of the older plates had a decided pink tinge and, therefore, for correct rendering of the colors a