In order to allow of modifying the illumina tion on the various parts of the negative by the interposition of suitably-shaped pieces of translucent paper, the printer is usually fitted with a sliding frame just underneath the glass negative bed. This frame is fitted with a piece of ground glass on which any shaped pieces of paper may be placed as desired. Besides regu lating the exposure times, nearly all printers allow of the illumination being varied at will by altering the distance between the lamps and the diffuser. In this case a number of suitably scaled positions should be marked out, corre sponding, for example, to illuminations which are double each other. Again, it is useful to be able to control the various white lamps separately,' or to move them in a plane parallel to the diffuser, so that the illumination in various parts of the negative can be relatively increased.
The beds of some printers are fitted with a number of stops which can be removed at will, and which allow a suitably-sized sheet of sensi tive paper to be adjusted to such positions that six or twelve pictures can be made in regular succession on it from the same negative. If the exposure is automatically regulated by the printer, one can be certain that the prints will develop up identically, and consequently the images will be of the same depth. Other more complicated types are fitted with a movable carriage carrying the sheet of paper and which stops automatically in predetermined positions. Lastly, there are printers which are provided with spools for making separate prints from film negatives in the whole strip, the film being moved for each successive exposure.
Blurred patches, caused by defective contact, and often noticed on prints which have been made in a printer, are frequently due to insufficient pressure along the join between the two halves of the back. If there is excessive pressure on the sides of the latter, the defect is accentuated by a thick negative of very small size (Fig. 18i). This difficulty can be overcome by placing the negative in a cardboard frame of about the same thickness as the negative. Blur which occurs in different positions in successive prints from the same negative is often due to air pockets formed between the sensitive paper and the negative when the pressure pad is not slightly convex. This fault can often be avoided by passing the back of the paper over a hard. wooden edge in such a way as to make the sensi tive surface slightly convex.
507. Light-sources for Positive Printing. Printing on print-out papers is nearly always done by the amateur in diffused daylight, but the control of printing is complicated to a great extent by the considerable variations which often occur in daylight from one minute to another. As normal production must be always
possible, even in very dull most large establishments usually make use of arc lamps (preferably arc lamps consuming So to 90 volts, i.e. enclosed arcs) or mercury vapour lamps (§ 294), as sources of light for printing. In such cases frameworks are pro vided to take a fairly large number of frames arranged symmetrically round the lamp. In this way nearly the whole quantity of light emitted is utilized.
Printing on development papers should be carried out with much less powerful lamps. 2 Incandescent electric lamps are generally used for the purpose, and according to the sensitivity of the papers usually employed their intensity should be neither too great nor too small. In this way both very short exposures, which are difficult to time and repeat correctly, and long exposures, which slow down the work, are avoided. The fact that the distance between the lamp and the negative has a considerable influence on the illumination (§ 13), and conse quently on the exposure, should not be over looked. To avoid too considerable local varia tions in illumination, the distance between the source of light and the negative should never be less than the diagonal of the negative.
The voltage of the current supplied by electric light companies varies very considerably at different times of the day, being generally at a maximum in the daytime and at a minimum at dusk. As variations in the voltage consider ably influence the visual intensity of the emitted light, and still more its actinic intensity 292), it is advisable to keep a voltmeter in the printing room, so that, as the result of a few tests, the volt-meter can be graduated in terms of equiv alent exposure, arbitrarily assigning the value x to the exposure corresponding with the normal voltage specified by the supply company.
For a small number of exposures on slow sensitive material (gaslight papers, warm-tone plates), the light afforded by the burning of a suitable length of magnesium ribbon is some times used. The ribbon is fixed on the end of a needle and lighted with a spirit lamp. The light of the latter has very little photographic action and can be left in a fixed position so as to mark the distance chosen for the combustion, the magnesium ribbon being removed from the flame as soon as it has begun to burn.