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HISTORICAL SURVEY V
after being exposed to the influence of light, appears the same as before the exposure. But if oil of lavender is poured over the coating of asphalt, it dissolves all the spots that have remained unchanged, and leaves behind all those that have been changed by light — that is, have been rendered insoluble. Thus, after several hours' exposure in the camera obscura, and subsequent treatment with ethereal oils, Niepce succeeded, in fact, in obtaining a picture. This picture was very imperfect, it is true, but interesting as a first attempt to fix the images of the camera, and still more interesting as evidence that there are bodies which lose their solubility in the sunlight. This fact was again made use of long after the death of Niepce, and it led to one of the finest applications of photography, that of heliography, or the combination of photography with copper-plate printing, which Niepce himself, to all appearance, had already known.
From experiments which have been recently carried out by Dr V. Vojtech it seems highly probable that this property of the insolubility of asphalt in turpentines after it has been exposed to light is entirely due to chemical changes, since no such action takes place when the asphalt is placed in an atmosphere of either nitrogen, carbon dioxide, or hydrogen during the exposure to light.
A Copper-plate Print. — A copper-plate print is produced thus : — A smooth plate of copper is engraved with the burin (or graving tool) — that is to say, the lines which should appear black in the picture are cut deeply in the plate. In producing impressions, ink is first rubbed into these cuts, and then a sheet of paper is placed upon the plate and subjected to the action of a roller press, whereby the ink is transferred to the paper and produces the copperplate impression. Niepce endeavoured to utilize light in producing these engraved copper-plates in place of the laborious process of cutting. To effect this he covered the copper-plate with asphalt, as before stated, and