In the second place we may take the case of a correctly exposed negative, but with which the development has been checked before the whole surface of the plate has darkened over. A print made from such a negative on one of the printing-out silver papers, if examined soon after printing has begun, will appear as a very delicate image, correct in gradation, but much too weak to stand toning and fixing. As printing proceeds the image will darken all over and it will be impossible to have pure whites. In this case it is necessary to strengthen the image proportionately throughout, that is, greater strength must be added to the high-lights than to the shadows. Undoubtedly the best method is that advocated by Mr. Chapman Jones and the formula is as follows : -

Mercuric chloride.........

1 ounce.

Distilled water.......

16 ounces.

Hydrochloric acid............

32 minims.

The negative is immersed in this till it is bleached white when examined from both sides and is then washed in several changes of water, acidulated with hydrochloric acid, say 40 minims to the pint. It should then be washed in a dozen changes of plain water when it is ready for the next operation, which is to blacken the bleached image. The formula is : Saturated solution of potassium oxalate 6 parts. Saturated solution of ferrous sulphate 1 part.

The ferrous sulphate solution should be poured slowly into the potassium oxalate solution, otherwise a yellow precipitate is formed. The negative is immersed in this until the image is thoroughly blackened when it should be well washed before drying.

If the increase in density is not sufficient the operations may be repeated as often as required, but many workers find that they cannot repeat the operations successfully, though this is entirely due to want of chemical cleanliness. Those who prefer to obtain considerable access of density by one operation may employ the following method, but the results cannot be guaranteed to be permanent. The negative is bleached and washed as in the previous method and is then blackened in a 1 in 20 solution of ammonia, after which a very thorough washing should be given before drying.

In the third place we may take the case of a negative which has been over-exposed. When the developer is poured over it the image will appear quickly, and before the high-lights have time to gain density the half-tones and shadows will also appear and in a few seconds the whole surface will darken over. If development is carried out very fully the negative will be dense, it will take a long time to print and the picture will be flat, showing but slight contrast between the lights and shadows. In such a case the density requires to be reduced more in proportion in the shadows than in the highlights. No chemical is known that will reduce the strength of the shadows without at the same time removing some portion of the density of the high-lights, but there is a whole host of chemicals which will reduce the density equally over the whole surface of the plate. In doing so it is obvious that the proportion removedfrom the shadows is greater than that from the high-lights and the desired object is attained. A simple and satisfactory reducer of this nature is : -

Potassium permanganate

4 grains.

Sulphuric acid...........

8 minims.

Water.........

5 ounces.

In this the negative is immersed till the required reduction takes place, when it should be well washed and fixed in a freshly-made hypo bath of ordinary strength. If after the permanganate bath the negative is found to be stained brown or yellow, the cause is an insufficiency of acid in the bath. The stain may be removed before fixation by immersing the negative in :-

Sodium Sulphite.............

60 grains.

Oxalic acid................

12 grains.

Water..............

1 ounce.

The negative must be thoroughly washed after this bath and fixed in hypo as previously stated.

In the fourth place we may take the case of an over-exposed plate which has been removed from the developer before great density has been obtained. A print from such a negative will show little contrast at any stage of printing and as printing is continued, will quickly darken all over. The remedy is to increase the density of the high-lights without adding to the density of the shadows. This is quite possible, but requires some judgment in checking the action at the required stage. The image is first bleached in a mercuric solution and is then blackened in a solution of silver cyanide. This latter solution at first converts the image of white silver and mercurous chloride to the metallic state and then dissolves out the mercury. The action commences on the surface of the film and proceeds slowly downward. The result is that as the shadow detail lies on the surface it is first strengthened and then reduced to its original strength. A proportion of the half-tones and high-lights are also on the surface, but also go deeper into the film. The result of careful treatment is that by checking the action at the right time the increase of density in the lights and half-tones is only partially undone and a considerable accession of contrast is obtained. This process of intensification is known as Monckhoven's and the working details are as follows. Three solutions have to be made up, they all keep well and they may be used several times until exhausted.