A NEW POWER IN PII()TOGRAt'tIY.
Just when to stop the development of the photographic negative on a gelatino-bromide plate ha.s always been more or less of a problem even to tile expert ; and as over-is more easily corrected than under-development, it has been the practice, when in doubt, to carry it beyond what was known to be necessary, trusting to reduction to bring the image back to the required density.
For this purpose various methods or reducing agents have been employed, but hitherto they have all had one fault in common--the altering of the values, tonality, or gradatim~, the most important feature of a negative. This they do in consequence of the fact, hitherto supposed to be inevitable, that reduction goes on equally all over.the plate, as much being removed from the delicate detail, in what will be the shadows, as from the denser deposit of the half-lights and lights, resulting in negatives that give prints of the white and black or " soot and whitewash " variety.
Recently, however, the brothers I,umiC~re, to whom photography is already much indebted, have given to photographers a new power in the shape of ammonium pers.ulphate, a solution of which has the property of attacking only, or at least first, the higher and half-lights without touching the weaker deposits in the shadows, thus enabling them to reduce contrasts and secure such values or gradation as they may desire.
It will be evident that with one of the older reducing agents that reduce equally all over the plate, and the new agent which acts only on the denser parts of the image, the photographer may with confidence develop to any degree of opacity, knowing that he has the power, by reduction, to produce any degree of gradation that he may desire.
Hardly less of a problem, especially to beginners, has been how to secn.-e correct exposure ; and according to at least one expert, the solution is to be found in ammonium persulphate. It is well known that over-exposure tends to flatness. The negatives may have all necessary detail, but the lights and half-lights are so translucent as to give only weak, flat prints. If, however, ammonium persulphate in conjunction with a Bromide be added to the ordinary developing solution, any degree of contrast may be obtained, even to simple white and black, the degree being in proportion to the quantity of persulphate added. For this purpose W. B. Bolton recommends a solution of anlnlOUiUlll persulphate 2 5 grains and amnloninln bromide 5 grains ill I ounce of water, and a few drops added to the developer. The action will be slower, and the degree of contrast greater, in proportion to the quantity of solution added ; but a few e: ~3eriments will show just what that quantity st'.ould tlc for any reasonable amotmt of over-exposure.
It may be well to add that the new reducer is not the acid or hydrogensulphate, NH4ttSO~, somelimes called the persulphate, but tile true persulphate, Ntt4SO ~, said to be produced by electrolysis flora the hydrogen-sulphate, thus NH,HS()~ ~:-: NH4S()~ j-H, the atom of hydrogen being eliminated and the persalt formed at the negative electrode.
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