When a professional gets a fogged negative he can generally recognize the nature of the fog and put his finger on the cause. The young assistant, however, is apt to assume that all fog is caused by the action of light on the plate before or during development. He learns later, with surprise, that there are other kinds of fog which may occur even when the plate is handled in a perfectly safe light. He finds that, besides ordinary light fog, there are atmospheric fog, chemical fog, and the peculiar form of chemical fog known as dichroic fog.

Atmospheric fog, as its name implies, is actually present in the atmosphere, but it is often visible to the photographic plate when it is invisible to the eye. This happens whenever the moisture or dust in the air. reflects ultraviolet and violet rays, instead of the visible rays belonging to other parts of the spectrum. And so it happens that a landscape, which the photographer thinks is free from all fog, may really be bathed in a thick invisible ultra-violet mist which is faithfully recorded on the plate. This experience is familiar to all who have done much photography in the West among the snow-capped mountains. Distant mountains can often be plainly seen when it is impossible to photograph them on an ordinary plate without a light filter. The whole difficulty is very simply solved by the use of Panchromatic plates and a K 2 or K3 filter.

Chemical fog is of an entirely different nature. It is due to uncontrolled chemical action during development. The whole process of development is based upon the principle that the developer will reduce to metallic silver those particles of silver salt upon which the light has fallen, whereas the particles upon which the light has not fallen will remain unaffected by the developer. If a very strong developer is used, however, there is a danger that it will reduce the particles of silversalt, whether the light has fallen on them or not, and the metallic silver which is thus deposited evenly all over the plate is known as chemical fog.

It is sometimes said that chemical fog is due as much to the instability of the silver salts in the emulsion as it is to the strength of the developer. The emulsion of a modern dry plate, however, is seldom at fault - but the developer which suits one emulsion is not always suited to another. The wise professional sticks to the formula recommended by the makers of the plates which he uses. Even when he does so, he may get chemical fog if the developer is too warm, or if he forces development in any other way. Some plates will stand more heat than others without fogging; but it is a safe rule never to use a developer below 65° or above 70° F.

From An Eastman Bromide Print By Geo. F. Wettlin Newark, N. J.

From An Eastman Bromide Print By Geo. F. Wettlin Newark, N. J.

The established method of guarding against chemical fog is to add potassium bromide to the developing solution. It has been suggested that the bromide combines with the silver salts in the emulsion to form a double compound of silver, which does not respond so readily to the action of the developer, and that there is, accordingly, less risk of the developer reducing any particles of the silver salts upon which no light has fallen. This is only one theory out of many. Few subjects in photographic chemistry have aroused such fierce controversy as the part played by bromide in the developer.

The fact remains that potassium bromide, in small quantities, is an efficient protection against chemical fog, and, in larger quantities, is an efficient means of adding to the contrast and brilliancy of the negative.

The most curious fog of all is a peculiar form of chemical fog known popularly as green fog and scientifically as dichroic fog. It is found on negatives in the form of a stain, which is yellowish-green by reflected light and reddish pink by transmitted light. This characteristic explains its.scientific name - for dichroic fog means literally "the two-colored fog."

Its nature and its cause are not so easily explained. The general view is that dichroic fog is found only when some ingredient of the developer has the power of dissolving the silver salts in the emulsion. Silver bromide, silver chloride and silver iodide are all practically insoluble in water - but they are all easily soluble in ammonia or ordinary hypo, and are slightly soluble in sodium sulphite. And so this peculiar trouble was very common in the old days when nearly all plates were developed with pyro-ammonia, and it is still apt to occur when the developing solution contains hypo or an excess of sodium sulphite.

When the developer contains none of these solvents, the silver salts remain firmly embedded in the gelatine emulsion, but when any of these solvents are present, some particles of silver salt are dissolved out of the emulsion. If the salts react with the developing reagent while they are in this state of solution or semi-solution, the metallic silver is deposited on the surface of the plate in an extremely fine colloidal state. This deposit is what is known as dichroic fog. It is naturally heaviest in the shadows where there has been the least light action, because it is in these parts that the particles of unreduced silver salt are the most numerous.

From An Eastman Bromide Print By Geo. F. Wettlin Newark, N. J.

From An Eastman Bromide Print By Geo. F. Wettlin Newark, N. J.

The chief characteristic of di-chroic fog is that it is almost entirely on the surface, while chemical fog is a deposit in the body of the gelatine emulsion. Di-chroic fog may often be removed by simple friction, or by mechanical reduction, as it is called. Generally speaking, it is difficult to get rid of it except by some form of surface reduction.

Troublesome as these three varieties of fog undoubtedly are, ordinary light fog causes still more trouble because it is so common. It is sometimes due to plate holders and dark rooms which are not light tight, but it is still more often due to the use of unsafe dark-room lamps.

Fortunately photographers are now recognizing more and more that they cannot rely on ordinary red, yellow or orange glass or paper to intercept the actinic rays. Many of the dark-room lamps which used to be sold gave barely enough light to make a few objects visible, and yet they transmitted such a large proportion of violet and blue rays that any plate of moderate speed was almost certain to be fogged. Nowadays these old lamps are being rapidly replaced by Wratten Safe-light Lamps. The Wratten Safe-lights are made of glasses coated with gelatine of that precise color which will give the maximum of illumination and yet transmit only those rays of light to which plates are least sensitive; and the Wratten Lamps are so constructed that only reflected light is used. These two improvements being based upon scientific principles have practically eliminated light fog from professional dark rooms.