Volltext Seite (XML)
MARCH 27, 1885. J THE PHOTOGRAPHIC NEWS. 201 upon by the inventor, and some of the improvements have been patented, but the exact points claimed will not be known until the Specification is published. The subjoined sketch of two forms of the clip requires no explanation. What becomes of all the spoiled gelatine plates? There must be thousands accumulating—reproachful monuments representing so much wasted money and labour. Everything has its use—even old Champagne bottles, until recently the most worthless article in the market of unconsidered trifles—but has no one thought it worth while to collect waste gelatine plates, clean them, and offer them to the manufacturers to be coated afresh ? The children of one of the Metropolitan Board Schools were recently photographed. While the operation was proceeding, one of the officials of the Board chanced to visit the school, and was thereupon moved to express his displeasure at the “ waste of time.” He futhermore said that the Board disapproved of the photographing of Board School children in school hours, for the reason he had alleged, namely, waste of time. Can this be really true ? There is not a Board School in the metropolis which has not been photographed. The children are taken in classes with their teachers, and the time for each group does not exceed ten minutes. The pleasure and esplrt du corps which the photographs give certainly more than outweigh the ten minutes’ waste of time. It is to be hoped that the official spoke without authority, and that his objections arose from an undue sense of his own importance. GELATINE EMULSIONS, AND HOW TO MAKE THEM. BY W. E. DEBENHAM. NO. I. A description of the photographic processes in which gelatine is employed as the vehicle of sensitive compounds of silver, covers a much wider field than that occupied by collodion but a few years since. Not only have gelatino- bromide plates almost entirely superseded collodion for the production of negatives, but the variety and beauty of colour obtainable with chloride and bromide of silver sus pended in gelatine have given an impetus to the production of transparencies for the lantern, and for window decora tion ; whilst the recent achievements in the way of produc ing prints upon gelatine paper, both by direct printing, and by development after a rapid exposure, prove that even if these processes are not destined to supplant the old familiar one of printing upon albumenised paper, they at least contain what Dr. Johnson would have called the “ potentialities ” of doing so. Another, and now very considerable development of the gelatino-bromide process, is that of the production of enlargements upon paper, and yet another is the production of enlarged negatives upon paper somewhat similarly prepared. The writer proposes in the following pages to give a description of each of the typical processes, together with a formula, which, generally speaking, if not quite in every case, he has tried himself, and sufficient details to enable those quite new to emulsion making to enter upon what is found by many to be a most delightful hobby. Without further reference to those early processes, or suggestions for processes, in which gelatine was proposed to take the place of collodion as a vehicle for iodide of silver in connection with an excess of silver nitrate, we come in 1871 to the publication by Dr. Maddox of a method which has been considered the parent of the modern gelatino-bromide process. In this process nitrate of silver and an alkaline bromide are added to a solution of gelatine, and silver bromide is thus formed in that state of suspension known as an emulsion, and this is poured upon a glass plate and dried. The drawback to the employment of this simple method is that the nitrate of the alkali is liable to crystallize upon the surface of the film, and so cause an irregularity which may show in the negative, besides inducing a tendency in the gelatine film to frill, or separate itself, during development or the after processes, from the glass "plate upon which it has been spread. This tendency to crystallization and frilling will be much less with certain nitrates than with others; much less with potassium nitrate than with that of ammo nium ; therefore, in this process, bromide of potassium should be employed in preference to bromide of ammonium. It is also obvious that the smaller the quantity of nitrate in a given quantity of gelatine, the more readily that gela tine will hold the nitrate without its crystallizing, or caus ing any other injurious influence. Bromide of silver in gelatine is capable of existing in a great variety of conditions, some of which will give, with a certain amount of silver in a given quantity of gelatine, a far richer and more opaque-looking film than others. For the particular process now under consideration it is especially desirable that a condition of silver bromide should be formed giving richness and body, so that a small quantity of it (i.e., the silver bromide), in proportion to the gelatine used, will suffice. Of course, with the smaller quantity of silver bromide, there will be a smaller quantity of nitrate of potash for the gelatine to retain. In the following formula, although there is eight times as much gelatine as nitrate of silver used, the film will be
