A Powdery Deposit on the Films.
This sometimes occurs after ferrous oxalate develop ment. It also may occur in the fixing bath if the negative have not been sufficiently washed after the alum bath. The cause is different in the two cases. In the first case the deposit may be removed by placing the negative in a mixture of one pint of water to a quarter of an ounce of hydrochloric acid. In the other case it is to be avoided by thorough washing of the negative after it has come from the alum bath.
Halation.
Sometimes when particularly trying subjects are photographed a peculiar defect is produced which goes under the name of "halation" or sometimes "blurring." If there be a very bright part in a scene which is so dark in parts as to require a long exposure, halation will he visible round the bright spot. The case which we gave as an example of a scene from which a ghost image might he expected will do as an example of one which will possibly give halation also. Around the dense portions of the negative which represent the windows there will be found a halo of fog, extending to possibly only one-eighth of an inch, possibly to one or two inches. In the print this shows as a light halo, and the effect is very disagreeable.
Where subjects have to be treated in which there are very bright parts in close juxtaposition to deep shadows, the plate, before exposure, should be laid face downwards on a pad of clean blotting-paper, and over the back of it there should be brushed a solution of bitumen in coal tar naphtha. The precise strength of this solution is not
of importance, but it should be of such strength that when spread on glass the film is nearly opaque.
The application of the solution prevents reflection from the back of the glass plate. Before development the " backing " is rubbed off by the help of a rag dipped in coal-tar naphtha.
Solarization.
This curious defect is seen when the conditions men tioned as giving rise to halation are present in an extreme degree. The appearance is sometimes entitled "reversal of the image," and this term well describes it. The very brightest parts of the subject, instead of being represented by great density in the negative, are represented by comparatively transparent portions. In fact a positive is produced instead of a negative. In the clays of collodion dry plates reversal of the image was very common ; so much so that at times the sky would he a positive whilst the landscape was negative. With gelatine plates it only occurs in very extreme cases. If, however, for example, the sun be included in the negative there may he reversal. If it be suspected that there may be such the only pre caution which can be taken is to use a somewhat in creased amount of bromide in the developer and to keep the solution in very rapid motion during development.
Reversal may be produced if too much hyposulphite of soda be added to the oxalate developer to accelerate its action.