. Radiography and radio-therapeutics . rds 1 inch in each direction, and the radiograph taken. A perfectpicture of the area required should be obtained. There is one objection, however, to this method, and this is the obviousone that the lower coin (assuming the tube to be beneath the table) willcast a larger shadow on the screen than the upper one, and therefore, asboth coins are opaque, it is not possible to say when the smaller shadow isexactly in the centre of the lower one, as it should be if the view of the headis to be a strictly lateral one. To obviate this defect Trevelyan George has
. Radiography and radio-therapeutics . rds 1 inch in each direction, and the radiograph taken. A perfectpicture of the area required should be obtained. There is one objection, however, to this method, and this is the obviousone that the lower coin (assuming the tube to be beneath the table) willcast a larger shadow on the screen than the upper one, and therefore, asboth coins are opaque, it is not possible to say when the smaller shadow isexactly in the centre of the lower one, as it should be if the view of the headis to be a strictly lateral one. To obviate this defect Trevelyan George has substituted a lead ringfor the lower coin (again assuming the tube to be below the table). This isvery simply made by describing two concentric circles on a piece of thicksheet lead and cutting away the superfluous metal. Exact measure-ments are not essential, as ears difEer much in size, but a convenient size isobtained by making AB equal to 20 mm., and CD to 14 mm. (see Fig.),and a convenient thickness of sheet lead to use is li Instead of the upper coin a circular lead disc, cut out of the same sheetof metal, is used, a suitable diameter being 7 mm. Now, it is of the utmost importance that the two articles, the ring andthe disc, be placed symmetrically in the auricles, and as they difEer in externaldiameter, this raises a difficulty. In order to get over this, the diameterof the disc is artificially increased to that of the ring, to AB, by em-
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