PHOTOGRAPHIC SOCIETY.

Otago Daily Times, Issue 11925, 25 December 1900, Page 7

PHOTOGRAPHIC SOCIETY.

The last monthly meeting of the Dunedin Photographic Society was held in the society's rooms on Wednesday evening, the 19th inst. Quite a large number of pictures from negatives taken on the occasion of the last two Saturday afternoon excursions were 3IIOWU, especially 11 nice display of pictures of the opening of the yachting season. During the evening Mr Gamble, of the Otago Witness staff, described the rnani;er of making process blocks from photographs for the purpose of reproduction 111 the printing press. The property of bichromate ol potash to make gelatine and other colloid substances insoluble after being exposed to light, it was explained, was the basis on which the process rested. A gelatine film is impregnated with the bichromate in the dark, and, alter diying, is exposed to sunlight or 1 electric light behind a negative. Those parts of the film which lie behind the clear parts of the negative have now become insoluble, and the rest of the film in like manner,- according to the varying degree of density of the negative till the densest portions are reached, where the- light does not penetrate, and consequently i no action takes place. A film superposed on a I zinc plate was placed in a bath of. weak nitric | acid, the result of which was that the acid ate ' down into the zinc wheie not protected' by . the insoluble gelatine, and to produced what was wanted—an image in relief which can be printed from in the press. A photograph being practically a stain, an ordinary negative would be useless for this purpose, hence a negativo of another kind is required. This, it was said, is done by employing a screen of glass on which are ruled lines forming :i netivoik of fineness .according with the character ol the work. The screen is placed between the lens and the picture to bo copied, and so leaves a system of cross lines to break up the high lights and ' half tones. These lines may be readily noticed in a picture which has been printed from a process block. Mr Gamble also explained the ' - printing of pictures by the three-colour pro- ' : cess, which was invented a few years s^o, and ■has come into use in reproducing photographs in what strongly represents natural colours. ». number of.pictures of this kind executed by firms which excel in this class of work were shown, and the lecturer remaiked that three negatives were taken, each through a different colour of glass, and the inks used in printing oft the impressions were of corresponding colours. The blending of the inks gave the desired result—namely, a picture not unlike ; one m natural colours.