If there is insufficient room to use the lens with its axis horizontal, a worker with any manual skill could easily arrange for the camera to move in vertical slides, the original to be copied being placed directly on the floor.
736. The simplest way of ascertaining the parallelism of the planes of the easel and of the focussing screen (assumed to occupy the same position as will be occupied by the sensitive surface) when these planes are not horizontal consists in obtaining a mirror (surface-silvered glass ; or one surface of a sheet of glass may be coated with black varnish ; or a sheet of tinfoil may be stuck at its centre) of such size that it can be exchanged for the focussing screen.' First fix this mirror to the easel facing the camera. Remove the lens board, and, turning back the frame of the focussing screen, go behind the camera and, looking through it, sight the mirror with a rudimentary sighter (a sight can be taken along one edge of a flat ruler) fitted on an easily movable support such as a tripod stand, head-rest, etc., so that the image of the sight-line reflected by the mirror forms an exact prolongation of the sight-line itself. The sighter being then left undisturbed and fixed, the mirror is put in the place of the rear surface of the focussing screen with its reflecting face turned to the back of the camera. If the mirror is in a plane parallel with the easel the coincidence of the line of sight and of its image will still be seen. If this be not the case, the adjustment of the camera or of the easel must be modified until the sight-line coincides with its image in the two positions given to the mirror.' 737. Factors Affecting the Sharpness of Copies. A copy is perfect only if absolutely sharp and if the thicknesses of the lines are reproduced on the same scale as for the entire In the copy of a pen-and-ink drawing, for instance, if the lines are thickened by a few hundredths of a millimetre only, the effect is considerably heavier ; on the other hand, if the lines are finer the drawing loses all strength.
It is necessary to remember that stopping down the lens excessively may impair the sharpness instead of improving it (§ 53). The
better the quality of the lens, the larger the aperture which it is best to use. Apertures of diameters smaller than one-fiftieth of the exten sion should be avoided.
An original that is not perfectly dry when it is placed on the easel may shrink progressively under the effect of the heat of the lamps, and the sharpness may suffer in consequence. On the other hand, the heating of the layer of air between the lens and the original, if the lamps are too near the latter, may cause currents similar to those that are seen when sighting objects behind a flame (F. Dogilbert, 1909).
Vibrations of the floor may render it impos sible to obtain a sharp image 152). It is at least advisable to avoid walking around the camera during the exposure, and particular care must be taken not to knock against it, even if it is suspended.
The model must always be placed before a black background covering the whole of the field embraced by the lens, so as to decrease the risk of fog and the weakening of contrasts by successive reflections between the components of the lens (§ 57), and diffusion by the interior surfaces of the camera.
738. Choice of Sensitive Material. An increase of contrast always increases the sharpness of the images by merging in the white of the paper any unsharpness resulting from the various circumstances mentioned above. Originals which include only black and white (printed matter, wood engravings, lithographs, geometrical draw ings, pen-and-ink drawings) will therefore be photographed preferably on the special emul sions for process work. Originals in black on a tinted or stained ground, or those in colours (architect's or engineer's blue-prints, etc.) will be reproduced by means of process panchromatic emulsions with the aid of suitable colour filters (§ 222).
In the case of full-tone originals (and among these we must include pencil or charcoal draw ings and copperplate engravings, of which the lines are of unequal strength) slow emulsions will be required with a very fine grain, or lantern plates, but not the special emulsions of very great contrast which can reproduce correctly only a very short scale, even if development is not forced.