Two dishes should be provided, one containing diluted developer and the other plain There should also be a sheet of glass larger than the print, placed level on the table ; a measure containing developer somewhat less diluted and thickened by the addition of glycerine, sugar or gum arabic ; and several brushes of suitable sizes, according to the work in hand.
The print should be immersed in the dilute developer and transferred to plain water as soon as the image is completely visible. After rinsing, it should be placed on the sheet of glass, face upwards. The parts of the image which it is desired to increase in density are painted over with the less dilute developer by means of a brush. The print should be frequently immersed in water to avoid the edges of this local develop ment showing too evidently. From time to time the print is again immersed in the dilute developer, and possibly, development may be completed with the print on the glass plate ; any parts which arc required to remain light are painted over with a weak solution of bisulphite which prevents their further development.
This method is only possible if the solution contains no developer liable to produce veiling from aerial oxidation (§ 339). Consequently, all developers containing hydroquinone are in admissible.
The development of very large prints can be carried out, in the absence of very large dishes, in the following manner : The print should be thoroughly wetted and laid on a flat support of suitable size, e.g. a bench covered with rubber sheeting. Developer, diluted with an equal volume of water, to render its action slower, should be quickly spread over its surface by means of a sponge or a large brush.
(c) POSITIVE TRANSPARENCIES (BLACK TONE) ON PLATES AND FILMS 567. General Considerations. Transparency emulsions being distinctly more rapid than those for bromide papers, all manipulations should be carried out in either orange-red or green light.
In addition to special transparency (lantern) plates, ordinary negative films or plates may be used for transparencies occasionally. Old plates of doubtful quality may be utilized in this manner, but it is generally impossible to obtain on these plates images as vigorous or possessing such good quality as those given by plates specially prepared for transparencies.'
Printing by contact on plates should be done in a printing-frame, and at a sufficient distance from a small source of light to avoid blurring through local want of contact. 2 If the edges of the sensitive plate are not protected by the rebate of the frame from the access of light, a mask should be used (§ 512). The mask itself should be attached to the glass side of the negative. Printing can also be done either by enlargement or reduction.
When the negative possesses strong contrasts, or when development of the transparency is prolonged in order to obtain great contrast, it is best to guard against halation (§ 231) by applying to the back of the plate an anti-halo paste or adhesive (§§ 236 and 238). There is no need for this precaution, as a rule, when printing on films, 1 568. There are two points to consider in making black-tone transparencies: the average density and the contrast desired.
The average density depends entirely on the exposure ; it should be greater or less, according to the method of examining the transparency and the strength of the illumination. For example, transparencies intended for projection in the home, by means of a comparatively weak lfglit, must be of less density than those for projection before a large audience by means of a lantern fitted with an electric arc or other high power light. 2 The contrast of the image depends entirely on development, the image possessing greater contrast in proportion to the duration of development. Too long development, however, produces too dense an image unless the exposure he somewhat under-timed. The contrast should lie greater or less, according to the nature of the subject. It ought to be much greater for an interior subject than for a landscape, and greater for a landscape than for a portrait.
Excepting for the reproduction of diagrams or drawings in black-and-white, the whites of a subject should never be represented by bare glass, unless they represent a direct source of light or its reflection.