. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. the intensity of the light, so that in the highestlights all but the portions under the largest opaqueobstructions are filled up, whilst these are closed tosuch an extent as to produce a very fine dot. A verysmall stop and long focus lens has to be used in orderto restrain the spreading action. A more promising kind of irregular grained screen,having, moreover, some peculiar optical features,is the Metzograph, invented by Mr. J as. Wheeler. Ithas absolutely no opaque grain on it, presentingt
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. the intensity of the light, so that in the highestlights all but the portions under the largest opaqueobstructions are filled up, whilst these are closed tosuch an extent as to produce a very fine dot. A verysmall stop and long focus lens has to be used in orderto restrain the spreading action. A more promising kind of irregular grained screen,having, moreover, some peculiar optical features,is the Metzograph, invented by Mr. J as. Wheeler. Ithas absolutely no opaque grain on it, presentingto the naked eye simply the appearanee of a pieceof ordinary transparent glass. On examination undermagnifying power, and at a suitable angle to the light,it is seen that the surface is covered with prismatic c 34 THE HALF-TONE PROCESS. reticulations, the character of the grain being in manyrespects similar to collotype (fig. g). It is statedthat the process of production of this screen consistsessentially in the sublimation of pyrobetulin, which,being deposited on the glass in a reticulated film of. Fig. 9. any desired fineness, is afterwards treated with hydro-fluoric acid, whereby a like character of reticulationis imparted to the glass itself. The effect of theprismatic reticulations is to divert a portion of thelight which reaches the screen, causing alternate lightand dark spaces which have consequently the same THE SCREEN. 35 effect as opaque obstructions, but without the harsh-ness which the latter seem to impart. Every toneof the picture will exert a different selective effect onthe grain, and, consequently, a truer naturalness oftexture and gradation is secured. The procedure forthe production of the negative is similar to thatfollowed with the lined screen, but the plate shouldhave an exposure not exceeding one-sixth of the timenecessary with a lined or pigmented grain screen. Asmall stop has to be used for the exposure, and thescreen must be placed as close as possib
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