International Review of Educational Cinematography (Jan-Dec 1931)

1034 When it is a question of negatives obtained by a sufficiently lengthy exposure and developed with pyrocatechine with caustic alkali, the residuant image is of a yellowish colour, sufficiently opaque to yield positives on paper. It cannot, however, be said that in this method of treatment the products of the oxidation of the de veloper exist in complete form. The combined action of cyanide and red prussiate cannot fail to sensibly influence the coloured product. In case of re dissolution of the silver by a more energetic oxidate then red ferrocyanide from the acidified permanganate of sulphuric acid the secondary images almost, or quite disappear. If comparative studies are made or negatives developed with ordinary developers (hydroquine-metol, diamedophenol, glycine) no trace of the image remains after the treatment with cyanide and red ferrocyanide. The pyrogall used with a moderate quantity of sulphite tends to give a vaguely yelow-tinged fog instead of a distinct image. Importance of the formation of the Secondary Image in the Use of the Latent Image. It is evident that if in the revelation of the latent image the composites of bromide of silver, and not only the reduced silver, are exploited the final effect is a better utilisation of the latent image. Indeed, while on the one hand thrreduced silver is formed, on the other there is the action of the bromine on the developer which results in the formation of a brome derivative or oxidate combination. The brownish yellowcolour of the latter is added to the black of the silver, making the image considerably more opaque. Snapshots of i/iooo, on plates or films, comparatively handled by cutting them in half and developing one half in an ordinary bath of hydroquinone and metol with desensibilisers of green pinacryptol, and the other half in a bath of pyrocatechine with caustic alkaline, yield considerable differences of opaqueness which may particularly be observed in the development of the positive. The importance of the absence or Sulphite for the clearness of ths Image. When a negative is developed with ordinary developers, above all, if no desensibiliser is used and more particularly with extra rapid plates, a tendency to the formation of a fog is noticeable which in our opinion is due to a general reduction of the silver bromide dissolved by the sulphite of sodium. On the other hand, by development with pyrocatechine without sulphite and without the desensibiliser (which cannot be used for the reasons above mentioned) very clear negatives are obtained, even with speedy exposures. The Grain of the Image of Negatives developed with Pyrocatechine. By comparing negatives developed in an ordinary bath of hydroquininemethol with others developed in baths of pyrocatechine with caustic alkali and without sulphite, we have gained the certainty that the latter offers considerable advantages. For the moment we have only made photomicrographs, moderately enlarged, and have limited ourselves to comparing them with negatives developed with hydroquinone metol. We intend to make more elaborate investigations by enlarging the photomicrographs on a larger scale and, above all, by comparing the action of the pyrocatechine bath without sulphite to that of the same bath containing a normal quantity of sulphite. We are further inclined to think that the reduction of the grain is principally due to the absence of sulphite, which does not, however, exclude that the secondary image may influence the grain