The advance of photography : its history and modern applications (1911)

86 THE ADVANCE OF PHOTOGRAPHY plaster is poured over it. This soon hardens, and gives an impression of the gelatine relief, being in high relief where the gelatine is hollowed, and vice versa (fig. 21). It appears as if a plate might be easily obtained for letter-press from such a gelatine relief. Let the gelatine film be exposed under a drawing. The black fines then keep back the light ; accordingly, the gelatine particles come out in high relief on being wetted with water. The drawing is therefore represented in high relief, and this is exactly what the printer requires ; nothing further would now be required than to cast the relief in plaster, and recast the plaster form in metal, as happens daily in the stereotyping of woodcuts. But unfortunately this process breaks down, owing to a trifling circumstance — the lines in the wet relief do not swell to the same height. But the printing-press requires them to be perfectly level, otherwise they cannot be equally inked and printed. Fif. 21. On the other hand, the casting can be very well utilised as a picture in relief, if suitable retouches are given to it. Reliefs of this kind with portraits were sold some years ago for seals, but the execution was very imperfect, and therefore they soon lost favour. But reliefs can be obtained in another way from an exposed gelatine film — namely, by hot water. As we have seen above, this dissolves the parts which, not having been affected b}T light, have remained soluble, while the parts affected by light, and therefore insoluble, remain untouched. These parts that have remained insoluble stand out as prominences. Another precaution is necessary. Suppose that N (fig. 22 a) is a negative, that c c are its opaque parts, and b the semi-transparent, which are called half-tones. If a film of prepared gelatine g (fig. 22 b) is exposed under