. Radiography and radio-therapeutics . liser. Position of tube for second exposure. (A. E. Dean.) 178 SWEETS METHOD OF LOCALISATION 179 which comprises a head-rest for securing the head of the patient above aplate-holder, so that plates can be changed and one-half exposed withoutdisturbing the patient. The pneumatic cushion shown is placed betweenthe patients head and the plate-holder. In use, the patients head is placed in position and the indicator shownon the right-hand side is placed in exact alignment with the centre of thecornea. The indicator is then pushed gently up to the eye itself,
. Radiography and radio-therapeutics . liser. Position of tube for second exposure. (A. E. Dean.) 178 SWEETS METHOD OF LOCALISATION 179 which comprises a head-rest for securing the head of the patient above aplate-holder, so that plates can be changed and one-half exposed withoutdisturbing the patient. The pneumatic cushion shown is placed betweenthe patients head and the plate-holder. In use, the patients head is placed in position and the indicator shownon the right-hand side is placed in exact alignment with the centre of thecornea. The indicator is then pushed gently up to the eye itself, and whenjust touching it is released and carried back by a spring exactly 10 milli-metres. Two exposures are made, the first with the anti-cathode of theX-ray tube in the same plane as the plate, and with one-half of the platecovered (Fig. 149). The unexposed half of the plate is then brought intoposition, and a second exposure made with the tube slightly tilted (). After development the positions of the foreign body and of the. Fig. 151.—Radiograms showing the developed plate of two positions. (A. E. Dean.) indicator are plotted upon a special chart sheet, a number of which aresupplied with every instrument. It should be noted that it is not necessaryto place the tube at any known distance from the plate, or to move thetube an exact distance for the second exposure. The special chartsprepared on squared paper show exactly the relative position of theindicator and foreign body and the position of the foreign body in the orbitor eyeball. The exact interpretation of the results obtained on the two plates isof some importance, and the two pictures in Fig. 151 show the orbit, with ineach a shadow of the foreign body. In the negative obtained in the firstposition (right half) the cone and ball are superimposed and show us a singleshadow. In that of the second position (left half) the cone and ball have aseparate identity. It is on the relative positions of the shadows of th
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