The precautions to be adopted in keeping stocks of sensitive paper are the same as those already described for keeping sensitive plates and films (§§ 280 and 281).
550. Exposure. Excepting for compensating for any inequality in a negative (§ 515) the exposure should always be made to a uniform light, obtained by a suitable spacing of several lamps in relation to the negative ; or a very satisfactory approximation can be secured by the employment of a single lamp sufficiently far from the negative.
When, for example, four lamps are used placed at the four sides of a square, at a distance equal to half the diagonal of the square, the variations of the illumination of the square will not exceed 4 per cent of the maximum illumina tion—variations which may be regarded as negligible.
When a single lamp is placed opposite the centre of the surface to be illuminated, the lighting decreases progressively from the centre, on account of the increasing obliquity of the rays of light. A variation is generally disre garded when it is not more than 15 per cent between the illumination at the centre and that at the most distant points ; this variation corre sponds with an obliquity of irti° for the extreme rays in the case of a source consisting of a point of light. In practice, the lamp is placed at a distance of about twice the longer side of the negative, but this distance may be appreciably reduced if a diffused light is employed, e.g. an electric lamp with an opal bulb.
In order to secure even Illumination of large surfaces with a battery of mercury-vapour lamps they should have a length equal to about one and-a-half times the greatest length to be illuminated, The tubes should be arranged parallel to each other, and parallel to the direc tion of the negative. They should be separated from each other by a distance of one and three quarter times their distance from the negative.
551. Bearing in mind that the illumination decreases very rapidly in strength as the distance from the lamp becomes greater (§ 13), uniform results cannot be expected unless the negative is kept at a fixed distance from the lamp. This applies equally whether systematic tests are made for obtaining the best time of exposure, or whether a series of prints is required from one plate. This condition is secured automatically when a printing machine is used. When printing in a frame the most simple method of obtaining these conditions consists in placing the lamp on a table in a marked position, and drawing lines on the table to indicate various positions for the frame. On each one of these lines make a
note of the largest plate which can be illuminated sufficiently uniformly in that position, and also the relative values of equal times of With some printing machines for photography and cinematography, the lighting may be varied by means of a rheostat placed in the circuit of the lamps, or of each lamp. It is necessary to note that under these conditions the quality of the light will vary as well as the quantity, and in a manner very different from one lamp to another.
It has been clearly shown by various experi menters, and particularly in the laboratories of Ilford Limited (1925) in regard to the develop ment papers of their own manufacture, that it does not necessarily follow that in printing by a feeble light the contrast in the prints is increased. On the contrary, the contrast is frequently reduced, as is also the maximum density of the blacks obtainable, whatever may be the time of exposure to the reduced light. This quality of regulating the contrast in prints by variation in the light, often regarded as a general rule, is, at the most, a special property of particular development papers_ 552. Experimental Determination of the Ex posure. The success of printing on development papers depends essentially on two factors. The first is the choice of an emulsion suited to the character of the negative, and the second is the adjustment of the exposure between certain limits determined by the sensitiveness of the emulsion, by the illumination of the negative, and by the opacity of the densest part of the negative under which it is desired to obtain a grey, differing (however slightly) from the tone of the paper or from the plain The actual time of exposure ought not to be very short, unless some automatic exposing device can be adopted, as a slight error assumes relatively a considerable importance. At the same time, it should not be so long as to restrict unduly the number of prints produced. When the exposure is made by hand, the intensity of the light,' and its distance from the negative, should be so arranged that the exposures should be, as far as possible, from 10 to 30 seconds.