The rules already given for the use of Sulphate of Iron acidified with Nitric Acid, apply also to the Nitrate of Iron; the proportion of free Nitrate of Silver must be large, and the film of Iodide of Silver not too transparent.
In developing direct Positives either by Pyrogallic Acid or the Salts of Iron, the colour of the image will be found liable to some variation; the character of the light, whether bright or feeble, and the length of exposure in the Camera, affecting the result.
A Process for whitening the Positive Image by means of Bichloride of Mercury.—In place of brightening the Positive image by modifying the developer, it was proposed some time since by Mr. Archer to effect the same object by the use of the salt known as Corrosive Sublimate, or Bichloride of Mercury.
The image is first developed in the usual way, fixed, and washed. It is then treated with the solution of Bichloride, the effect of which is to produce almost immediately an interesting series of changes in colour. The surface first darkens considerably, until it becomes of an ash-grey, approaching to black; shortly it begins to get lighter, and assumes a pure white tint, or a white slightly inclining to blue. It is then seen, on examination, that the whole substance of the deposit is entirely converted into this white powder.
The rationale of the reaction of Bichloride of Mercury appears to be, that the Chlorine of the mercurial salt divides itself between the Mercury and the Silver, a portion of it passing to the latter metal and converting it into a Protochloride. The white powder is therefore probably a compound salt, as is further evidenced by the effects produced on treating it with various reagents.
SECTION II.
On Collodion Negatives.
As in the case of a direct Positive we require an image which is feeble though distinct, so, on the other hand, for a negative, it is necessary to obtain one of considerable intensity. In the Chapter immediately following the present, it will be shown that in using glass Negatives to produce Positive copies upon Chloride of Silver paper, a good result cannot be secured unless the Negative is sufficiently dark to obstruct light strongly.
The Collodion and Nitrate Bath for Negatives.—A Collodion containing a very small portion of Iodide and yielding a blue transparent film in the Bath is not well adapted for taking Negatives. Pale opalescent films often give too little intensity in the high lights, and, unless the Nitrate Bath be acid, do not admit of being exposed in the Camera for the proper length of time without cloudiness and indistinctness of image being produced under the action of the developer. The effect known as "solarization of negatives," i. e. a red and translucent appearance of the highest lights, is also more liable to occur when operating with a very pale film. On the other hand, if the layer of Iodide be too yellow and creamy, the half-tones of the image will often be imperfectly developed, so that a middle point between these extremes is the best.
A pure and newly prepared Collodion, although highly sensitive to light, does not always give, with one application of the developer, a sufficiently vigorous image to serve as a negative matrix; and this particularly in the most brightly illuminated parts, such as the sky in a landscape photograph, or the white borders of an engraving. But on keeping the Collodion for some weeks or months it becomes yellow, if iodized with the alkaline iodides, and a decomposition takes place in it, as before shown ([p. 97]), which lessens the rapidity of action, but adds to the intensity of the negative.