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20 THE ADVANCE OF PHOTOGRAPHY This process made relatively little progress for more than thirty years after the experiments of Talbot. The explanation of this fact is, no doubt, to be found in the circumstance that the paper prints were, as then made, not very distinct, being often rendered worthless by spots and stains. Another reason is that the preparation of the paper required especial care, and therefore frequently failed to give good results in the hands of the inexperienced. Further, the papers prepared according to this method soon spoiled, and had on that account to be used imme- diately after their preparation. These disadvantages were removed by the invention of Romain Talbot's lichtpaus paper, which was sold ready prepared and could be kept for months ; and by this means the process became much more widely used. Talbot's Paper for Camera Obscura. —Talbot, the in- ventor of this paper process, carried out further researches, in the endeavour to represent on paper, by the help of the camera obscura, objects which cannot be pressed upon sensitized paper—for example, a person or a landscape. He attained this object two years after Daguerre's discovery, by means of paper prepared with iodide of silver. He dipped paper in a solution of nitrate of silver, and then in a solution of iodide of potassium. He thus obtained a slightly sensitive paper, but one that could always be rendered very sensitive, by plunging it into a solution of gallate of silver. 1 When this paper was exposed to the light in the camera obscura, a picture was not at once formed—this was only clearly developed after lying some time in the dark, or by subsequent treatment with gallate of silver—but it came out as a negative, and not as a positive. Thus, for example, in taking a portrait, the shirt appeared black, also the face ; while the black coat, on the contrary, came out white. 1 The nature of this peculiar process is explained further on.