DEFECTS AND REMEDIES.
Of the defects which, for the sake of completeness, we mention as existing in plates, few will ever be come across in the present day, whilst we think we may say with confidence that none of them will be found in the Britannia plates, as every batch of these is tested, not only by ourselves, but by an independent photographer, before a plate is allowed to go to the public.
If the enthusiasm of the amateur at any time reach such a pitch that he determines to make his own plates, he is likely to meet with any or all of the faults which we describe.
General Fog.
When we described the development of a plate we said that small portions of the film should remain quite white, and after fixing should appear quite transparent. If there be no such transparent portions, but on the con trary every portion of the plate be more or less opaque, it is said that there is fog ; and according as the least opaque portions are of the same colour as the denser or are of a different colour, the terms "general fog," "green fog," etc., are applied. General fog may be of any degree of intensity, from the smallest possible veil over the shadows to such density that the whole negative is almost opaque. It almost always exists in a gelatine negative to a slight extent, and in fact a negative which shows absolute clear ness in any part is not so good as one which shows a very slight veil.
Apart from a defect in the plates rarely met with in commercial articles of the present clay, the fog must be due either to the access of light at some time or another or to development. Most probably it is due to the first mentioned cause.
Light may act in either of the three following manners : First, by an unsafe light in the dark room ; second, by a defect in the camera ; and third, by over-exposure.
If the plate after fixing be examined and it be found that the portions which are protected in the camera by the wires or rebates of the dark slide are clear, then we know that the fog-producing action is exerted in the camera, and must be either one or other of the following. There must be a defect in the camera, or the plate must have been over-exposed.
To judge which it is, the cap is placed on the lens ; the camera is placed in the most brightly-lighted spot which can be found, the ground glass is removed or hinged back, the photographer covers his head with the focussing cloth and looks for any hole which may admit light. A few seconds' examination is not sufficient. He
must keep his head under the covering till his eyes have become accustomed to the darkness before he can be sure that there is no defect. If there be one discovered it has of course to be made good. If there be none, the dark slide must be examined, and if this also be perfectly light-tight there can be no other assumption but that over-exposure has been the cause of the fog.
if it be found that the protected portions of the plate have fogged as well as the others, then it may be assumed that the fog is produced either by unsafe light in the dark room, by error in development, or is clue to defect in the plates.
To discover which of these it is we may develop an unexposed plate in total darkness. This is easily done, the operations being guided by feeling alone. The plate need be only developed and washed. Fixing is unneces sary. If there be still fog it is clue to either faulty plates or faulty developer. If there be no fog this time it may be assumed that the fog which was visible before was produced by unsafe light in the dark room. To corrobo rate this a plate is placed in the dark slide. The shutter of the slide is drawn half-way up, so that one half of the plate is exposed to the light of the dark room. The slide is placed in the position usually occupied by the plate during development, and is left there for three to five minutes. The plate is now developed. If the half which was unprotected by the shutter of the dark slide blacken, it is proof positive that the light in the dark room is not safe. The red and all other lights are ex tinguished, and the photographer looks for any chink or cranny which might admit white light. If he finds none the assumption is that he is using too much red light. It must be understood that dry plates are slightly sensi tive even to red light. An unlimited amount of this must therefore not be used. The lamp must be turned lower or the developing dish must be placed farther from it during development.