The wall in which the objective O (Figs. 3 and 4) is placed, as also the brick socle aa and ee (Fig. 2), on which the support for
Fig. 3.
Fig. 4.
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the original and the focussing screen rest, run on rails, and are absolutely isolated from the brickwork of the building in order to avoid any possible vibrations.
The stand for the original TT′ is provided with screw fittings, cams and wheels, which make it possible to raise the original, to lower it, to move from right to left, and vice-versâ, vertically and horizontally, so as to place its centre axial with the optical axis of the lens, as well as parallel to the focussing screen in the dark room. The whole of the mechanism lies at the back of the stand, so that there is absolutely nothing in front of the original.
The focussing table EE′ is constructed in a similar manner to the stand for the original, and is movable in every direction in the same way. It carries in front two wood clips, in which the board with the original is placed. The table for the original, like the focussing table, is constructed on rollers, which run on the already-mentioned rails. In order to be able to fix these as soon as a sharp focus is obtained a brake is fitted. The placing of the original table at the distance from the lens is accommodated according to the size in which it is to be reproduced, and has already been estimated. There remains, therefore, only to trouble about the fine focussing, which is very quickly effected.
For taking line or wash drawings and for enlargements up to 80 × 80 cm. a Steinheil wide-angle aplanat is used, which gives no distortion. The duration of exposure is with such subjects from eight to ten minutes. For smaller subjects, and principally for photographs on wood, an orthoscope by Voightländer is used, and the exposure varies from four to six minutes.