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332 THE ADVANCE OF PHOTOGRAPHY
important when photographs are being taken. The gap can be regulated when the image is being observed, by means of the attachment shown beneath the camera front.
The camera itself is adapted for the use of plates which measure 7 inches by 5 inches ; the greatest distance by which the focussing screen can be separated from the eye-piece is 28 cms., i.e. about 11 inches.
There is a very useful mechanical stage which can be attached to the stage T after the removal of the stage rings. By means of certain attachments to this stage, together with vernier scales, the exact position of any point of interest in a specimen of metal can be located, and it is thus possible to make a record of any points of special interest, and be able to return to them at any time.
By means of the next figure a much clearer idea of the arrangements of the stage and the attachments to the lamp will be obtained.
This diagram represents the arrangement of the apparatus when using a high-power magnification. The lamp (fig. 144) is placed in its lowest possible position, and then moved along the bench (fig. 144) until it is as far as possible from the stand. The iris diaphragm I (fig. 145) is next placed in position. The two pencils at right angles to one another in the extreme right of the figure represent the positions of the carbons of the electric arc lamp ; the carrier S2 having been moved towards the lamp, the iris diaphragm is closed down until it has only a, very tiny aperture.
To arrange the parts of the stage it is first necessary to loosen the screw r, so as to be able to lower the tube B. When this tube has been moved as far down as possible, retighten the screw r. The knob R1 is used for withdrawing the slide which holds the objective 0. This should be made use of when it is wished to place in a fresh objective. Now refer to fig. 144 ; note the side projection between the camera and the stage. This is