When it is desired to improve a very thin photographic negative so as to yield a fairly good picture, first ascertain whether the negative is thoroughly free from hypo. This is essential, no matter what process of intensification is employed. The negative may be tested for hypo by mixing with a weak solution of iodide of starch some of the final drippings from, the negative when it is removed from the washing water. If hypo is present in the film the colour of the starch will be discharged. Or, as a precautionary measure, the negative may be placed for a time in a. 2-per-cent. solution of anthion, which readily removes hypo from the film, and then well rinsed in water. To intensify a negative, a portion of the image consisting of metallic silver must first be converted into silver chloride, and to bring about this result the negative is placed in a solution (a saturated solution of mercuric chloride) from which chlorine can be absorbed. In this solution the negative remains until it is bleached white. The degree of bleaching governs the degree of intensification, but must not be overdone. Next wash the negative well for ten minutes to free it from any excess of mercuric chloride. The negative is then placed until it becomes black in a 10-per-cent. solution of sulphite of soda.

During the immersion the dishes containing both solutions should be rocked, to avoid uneven markings. The density, particularly of the lights, will be found to be considerably increased after the blackening of the negative. There are other methods of intensification, and the most popular of them (probably because a long range of effects is obtainable with it) is to blacken with ammonia, but the mercury and soda process described above is the one more likely to be successful in the hands of a beginner, as there is with this process a greater freedom from stains than with mercury and ammonia. Intensification is not necessarily permanent, therefore negatives that have been so treated require careful preservation. The process may, of course, be carried out in full daylight. Contrasts may also be forced up by printing from the negative on bromide paper.