As the last negative comprehends almost the entire base of the map, special attention was devoted to its conversion into a printing plate. The process employed is known in Germany as “Albertype” or “Lichtdruck,” in England as “collotype,” and in America as “heliotype” or “photogelatin.” Of these names, “collotype” seems to be the most fitting. Briefly, this process consists of sensitizing a gelatin film with a chromic salt and exposing it to light under a negative. In proportion to the amount of light passing the negative there will be a reaction in the chromated gelatin. In this reaction the gelatin loses its power of absorbing water and takes on the opposite property of holding “non-watery” substances, such as printing ink. The action of the light is a graded one, varying from full effect under the clear parts of the negative to nil under the very dense parts. A similar gradation in ink-retaining powers is acquired by the exposed gelatin film. Thus, where the light exerts full effect the gelatin will be completely hardened and will hold the ink in its greatest intensity (solid); the parts which receive less light or none at all will hold the ink in attenuated quantity. The lights and shades of the monochrome picture are thus reproduced. In order that the film may exercise this selective power of taking on or rejecting ink it is necessary that the unaltered parts be kept moist. Therefore, after exposure under the negative, the film is washed to free the gelatin of the unused chromates. While still moist it is rolled with a roller carrying printing ink. This roller will discharge its ink on the hardened parts of the film in proportion to the amount of light that each part has received through the negative. If a sheet of paper is then pressed on the inked film it will lift the ink and the resulting impression will be of the same character as the base color of the Leardo Map. It is of interest to note that as the light-affected and hardened surface of the film accommodates itself to the unaffected underlying gelatin (as the latter swells in washing) it breaks up into a net of lines. This reticulation is barely perceptible in the high lights of the picture but gradually increases in strength until the mesh fuses into the solid color of the deepest shades. It will now be apparent that the feasibility of printing these colloid plates hinges on the fact that the graded ink-attracting mesh is separated by inversely graded ink-repelling, interstitial, unaltered, and moist gelatin. Its mesh not being apparent to the unaided eye, the collotype approaches the fidelity of a true photograph in the rendering of details. For this reason, the collotype process has been selected as best suited for the reproduction of the Leardo base.