The student will find that for certain effects he may intentionally produce a slight fog over his plate, as has often been done with very good results; but if his plates are unintentionally fogged, they are ruined. Fog is due to light having had access to the plate, either during manufacture, during exposure, or during development. By developing an unexposed plate it can be proved whether it was fogged during the manufacture, as In that case the plate turns black. If the fog is caused by a leaky camera the edges of the plate, which are generally clear glass, are not fogged, for they have been hidden behind the rebate of the dark slide. Light coming through the dark slide shows itself in lines or patches, and is not general. If all these sources have been eliminated, the dark room must be suspected. This is tested by putting a plate in the slide, drawing the shutter out half way, and exposing the plate for a few minutes to the developing light. If the exposed half fogs, then the dark room is to blame.

Red Fog. Green Fog

We have only met with this phenomenon once, and that was in developing a uranium plate. This is green by reflected light, and red by transmitted light. It is generally deposited at the corners of the plate and round the edge.

Yellow And Brown Fogs

Are rarely met with, and are yellow and brown by refected light, whereas stains are coloured only by transmitted light. The student can easily distinguish between fogs and stains in this way. We have been very successful experimentally with Captain Abney's method of clearing off green fog. He recommends the following solution to be used alter fixing: -

The plate should be well washed after this treatment, and developed up with the ferrous oxalate developer.

But such plates are not always saved artistically by the method, for the tonality may be thrown out, and the texture of substances is nearly always damaged.

Frilling

Is due to the expansion of the gelatine, and will rarely occur if the plate be put in the alum bath before fixing. The gelatine can be made to contract by soaking in methylated spirits of wine.

Blisters

Are. of rare occurrence, and will dry out if the plate be carefully handled and washed in alum, as directed.

They may be treated locally with methylated spirit, which causes the gelatine to contract.

Dense Negatives

the best reducer we know of is Dr. Eder s. He recommends the use of - A., one part chloride of iron to eight parts of water. B., two parts neutral oxalate of potash to eight parts of water. A well-known authority on photographic matters, Dr. H. W. Vogel, says, " Both solutions keep a long time without deteriorating. Immediately before using, equal parts of A. and B. are mixed, forming a bright green solution, which keeps well for several days in the dark, but decomposes in the light. Of this mixture a little is added to a fresh and strong solution of' hypo,' In difficult cases 1 part 'hypo ' and 1/4 to 1/2 of iron solution are employed. The plate to be reduced is placed in this solution. The image weakens quickly and uniformly. The plate is taken out and washed just before the desired reduction is reached, because the action continues during the washing, gradually diminishing under the stream from the tap. This reducer acts on plates developed either with 'pyro' or 'oxalate,' and does not destroy the details in the shadows like cyanide. There is also less tendency to frill than with the cyanide bath."

Reducers, like intensifiers, should not be resorted to, unless in case of a very valuable negative, for it must never be forgotten that, though the printing density is reduced, the tonality is not corrected.

Yellow-Stains

Due to the developer, are easily removed by Edwards' clearing solution, which we have found most effectual -

Transparent Spots

Are due to dust in camera or slide, or to using the "hypo" bath too long. If the spots have sharply defined edges, they are due to air bubbles forming at the beginning of development.

Halation

This is a bug-bear we have had little experience of, though we have taken many interiors. The only occasion on which we met with it was once when the plate was overexposed on a stained glass window, containing much blue in it. If a large stop be used, and the exposure kept as short as possible, our experience is that no halation need occur. If, however, the student fears it, and there is always a danger of it where any bright lights act on the film, he should, with a squegee and some glycerine, apply a piece of some dark tissue to the back of the plate; this is easily stripped off before development.