The development of direct Positives.—With Sulphate of Iron as a developer, it is most simple to develope the image by immersion. The solution may conveniently be poured into a vertical trough, such as that used for exciting, and the plate immersed by means of a glass dipper in the usual way. Unless the weather be cold, the image makes its appearance in three or four seconds, and the film is then immediately washed with clear water. Whilst in the Bath, the plate is kept in gentle motion, and the operator must not expect to see the image very distinctly, except the high lights; the shadows, being faint, are partially concealed by the unaltered Iodide, but they come out during the fixing. The action of the Sulphate of Iron is stopped at an early period, or an excess of development will be incurred. The Bath may be used repeatedly.
In using Pyrogallic Acid or Nitrate of Iron to develope glass Positives, the plate may be placed upon a levelling-stand, or held in the hand, or by the pneumatic holder, and the solution poured on quickly at one corner; by blowing gently or inclining the hand, as the case may be, it is scattered evenly over the film before the development commences.
If any difficulty is experienced in covering a plate evenly with a strong developer before the action commences, it may be overcome by using a shallow cell formed by cementing two or three thicknesses of window-glass on a piece of patent plate to the depth of a quarter of an inch. The size of the cell should be only slightly larger than the plate intended to be developed, that the waste of fluid may be as little as possible.
The cell is held in the left hand, and the plate being placed in it, a sufficient quantity of the developer is poured on at one corner. By a slight inclination, the fluid is caused to flow in a uniform sheet over the surface of the film, backwards and forwards. The image starts out quickly, and the developer is then at once poured off, and the film washed as before.
It is very important in developing Positives to use a sufficient quantity of the solution to cover the plate easily; otherwise oily stains and marks are formed, from the developer not combining properly with the surface of the film. For a plate five inches by four, three or four drachms will be required, and so in proportion for larger sizes.
The appearance of the Positive image after developing, as a guide to the proper time of exposure.—When the plate has been developed, it is washed, fixed, and laid upon a dark ground, such as a piece of black velvet, for inspection.
In the case of a portrait, if the features have an unnaturally black and gloomy appearance, the dark portions of the drapery, etc., being invisible, the picture has been under-exposed.
On the other hand, in an over-exposed plate, the face is usually pale and white, and the drapery misty and indistinct. Much however in this respect depends upon the dress of the sitter (see [p. 66]), and the manner in which the light is thrown; if the upper part of the figure is shaded too much, the face may perhaps be the last to be seen. The operator should accustom himself to expend pains in the preliminary focussing upon the ground glass, and to ascertain at that time that every part of the object is equally illuminated. For this reason, pictures taken in a room are seldom successful; the light falls entirely upon one side, and hence the shadows are dark and indistinct.
The development of Negative Pictures.—This process differs in most respects from that of Positives. In the latter case, there is a tendency to over-develope the image; but in the former, to stop the action at too early a period; hence it is common to find Negative Pictures which are insufficiently developed, and too pale to print well.
In developing Negatives, many operators place the plate upon a levelling-stand, and distribute the fluid by blowing gently upon the surface; others prefer holding it in the hand and pouring the fluid on and off from a glass measure. The quantity of developer required will be less than that used for Positives, inasmuch as, if the Acetic Acid be present in sufficient excess, it is easy to cover the plate before the action begins. Some Collodion however, especially the glutinous kind, seems to repel the developer and prevent it from running up to the edge of the plate. When this is the case, or when oiliness and stains are produced, from the Bath being old and containing Ether, Alcohol must be added to the solution of Pyrogallic Acid.