Dulness in a photographic negative is generally due to fog, caused by over-exposure, incorrect development (i.e. using too much No. 2), or accidental exposure to light. If the negative is fairly transparent, soak it in water for a few minutes, and then immerse it in a saturated solution of bichloride of mercury until it begins to bleach to a cream colour. "Wash for fifteen minutes, and then place in a clean dish, film up, and pour over it a solution composed of 2oz. of water and thirty minims or drops of strongest liquor ammonia. The negative will instantly turn black (or it should be allowed to remain until it does). This operation is called intensifying; it increases the density and contrast of the negative. As the ammonia solution in careless hands is liable to produce stains, due to insufficient washings, some people prefer to use a saturated solution of sulphite of soda, with which very little washing is required between bleaching and blackening; but the density obtained is much less, partly owing to the blue colour of the deposit. Allowance for this should be made by bleaching thoroughly.

The following formula may also be used after thorough washing, and gives a red image of great contrast: - Uranium nitrate 100 gr., potassium ferricyanide 10)gr., acetic acid 1/2 oz., water 10oz. Rinse only and dry. Wetting the negative, pressing it between blotting-paper to absorb surface moisture, immersing in methylated spirit for ten minutes, and then drying by gentle heat, clears the shadows and gives greater contrast. Of course, if the dulness arises from a want of sharpness, the above is of no service, and the only remaining plan is to work over it with the retouching pencil, but this is a long and tedious process in most cases.