ADJUSTING THE GLASSES.
A peculiarity of the Cooke lens in which it differs from others is its capacity for ad necessary to obtain flatness of field and freedom from distortion are shown by Fig. 552. A number of test objects, fastened on the wall, are in a plane normal to the axis of a special tramway camera. The latter runs on rails which guide it truly, and the focussing is clone within the camera itself. Images of the test justment. As shown by Fig. 551, it con sists of three simple lenses. The middle glass is held by screws which are used in the final assembling to set it perfectly in agreement with the other two, this work being done by the aid of the instrument just described. By its means the lens is adjusted so that it produces sharp de finition when turned in any direction, and so that the image seen in the microscope remains stationary while the lens is rotated upon its axis. When this is at
tained, it is certain that it will give sharp definition at any part of its field, hut it is not yet assured that the image is flat and free from distortion. Adjustment in these respects is obtained by varying the separation of the glasses, without disturb ing the axial adjustment already made. Appliances used for the adjustments objects are magnified and examined in the focal plane, and, by a special device in the camera, charts are drawn to record the form of field and the astigmatic cor rections of any lens tested. The same camera contains a means for measuring distortion, a colour test is applied by another attachment, and the prismatic support in the rear is an extension cap able of accommodating the largest photo graphic lens ever made.