. Radiography and radio-therapeutics . ig. 154) contains inits interior a cross wire and a smallreflecting mirror, and is adjusted bya screw. On looking down the eye-piece, one adjusts the telescope untilthe reflecting mirror shows the crosswire to be exactly tangent to the sum-mit of the cornea ; the metal indicatorwill then be 10 mm. from the centre ofthe cornea of the eye under examina-tion. A small incandescent lamp isattached so that the adjustment may bemore readily observed. The positionof the indicator is further verified bymeans of a small hole in the overheadmirror (Fig. 155). By an
. Radiography and radio-therapeutics . ig. 154) contains inits interior a cross wire and a smallreflecting mirror, and is adjusted bya screw. On looking down the eye-piece, one adjusts the telescope untilthe reflecting mirror shows the crosswire to be exactly tangent to the sum-mit of the cornea ; the metal indicatorwill then be 10 mm. from the centre ofthe cornea of the eye under examina-tion. A small incandescent lamp isattached so that the adjustment may bemore readily observed. The positionof the indicator is further verified bymeans of a small hole in the overheadmirror (Fig. 155). By an ingeniousdevice, when the patient looks atthe reflection of the indicator in themirror above with his iminjured eye,the optical axis of his injured one is |brought parallel to the plate (). The X-ray tube is centred bymeans of markings on its lead glassshield, and the first exposure is madeon part of the plate, the remainderbeing protected by a metal shutter(Fig. 154). A metal shutter is now placed over the exposed portion, and. M^: Fk;. 155.—Diagram of the optical sjsteni.(Dr. A. W. Salnioud.) Y, vertical image of ball as seen byuninjured eye ; M, mirror ; X, injured eye ;Y, uninjured eye. 184 RADIOGRAPHY the tube, carrying with it the indicator and the plate, is moved a definitedistance towards the patients feet and the second exposure at once madeon the previously protected portion of the plate.
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