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Phic Process (Let us suppose that a map has)

Let us suppose that a map has been compiled and drawn with great care, and that it is desired to multiply copies of this original in the lithographic process. The first step in the process is to obtain a negative; for which purpose the map is placed upright upon a plane-board, and the camera opposite to it at such a distance as to give the desired ratio between original and co negative is now taken on glass coated with collocopy. A dion in the usual way, observing the greatest care to avoid distortion of all kinds, and to produce a negative of the highest excellence, success in which depends entirely upon the knowledge, sheet of plain, positive photographic paper is now coated on one side with a mixture, consisting of gelatine, softened and dissolved in water, to which a quantity of bichromate of potash and albumen has been added. The paper, evenly covered with this fluid, is dried in the dark, when it will be found possessed of a smooth glassy surface, and a bright yellow color. This surface is still further improved by passing it through the press in contact with a polished plate.

A suitable piece of positive photolithographic paper thus manufactured is now to be exposed to the action of the light under the negative of the map already described. This is accomplished in an ordinary pressure-frame, the time required varying from 10 to 15 seconds, or several minutes, according to the brightness of the weather. The positive thus obtained presents itself to the eye as sheet. a brown drawing upon the clear yellow of the is covered all over, while dry, with transfer-ink, The exposed photographic copy of the original which is accomplished by running it through the press with its face in contact with a stone which has already received a coating of such ink. After it is separated from the blackened stone it will be found to have brought away with it an evenly distributed film of inky matter, forced by the pressure into intimate contact with the unexposed, as well as the exposed portion of the surface. This operation is known as "blacking" the positive print; that now to be described is called "coagulation," its object being to effect a change of that nature upon the albumen contained in the coating of the organic matter. For this purpose moisture and heat are necessary, and both are applied very simply, by letting the blackened photographic copy swim upon the surface of boiling water with wet that with hot water. After the lapse of a cerits inky side upwards, for it is important not to tain period, determined by the experience of the operator, he proceeds to the next step in the process, that of "washing off." For this purpose the print is laid upon a smooth surface, such as a plate of glass or porcelain, and friction with a wet sponge, or other suitable material, is applied to the black inky coating, under which the photographic image still exists, and to develop which is now the object in view. The operator soon becomes aware that the moisture which percolated through the paper from the back, has exerted a tine in the sensitive coating; it has caused it to softened or gelatinizing influence upon the gelaswell, and to let go its hold upon the ink. But this change does not extend to those parts of the words, to those places which were unprotected by coating which were acted on by light; in other the opacity of the negative; they remain intact, uninfluenced by the solvent or moistening effect of the water. Accordingly the operator finds a under his hand as he continues the friction. This facsimile of the original map gradually develop removed, save those required to form the picture, process is proceeded with until all traces of ink are which must be clear and distinct in all its details.

Abundance of hot water is then poured over it, so it is then finally dried, which completes its prepaas to remove every particle of soluble matter, and ration.

A stone to which a fine smooth surface has been lithographic press; upon this is placed (inverted) imparted, is now slightly warmed, and put in the the positive print, after it has been damped by lying between moist paper, and the whole is then passed repeatedly through the press. On examination the paper will now be found to have attached itself firmly to the stone, so that some | prepared and exposed to the action of the light. force is required to separate the two. When the former is removed it brings with it its albuminous coating, which gives to it while damp a parchmentlike appearance; but the ink is gone: it has left the paper for the stone, and on the latter we find a reversed drawing of the map, one which, after it has been properly "prepared," will print as well as if it had been drawn by hand.