. Practical radiography : a handbook for physicians, surgeons, and other users of X-rays . 72. placed on the body, and fromit the point of incision markedon the skin (Fig. 73). The same principle isutilized in a more practicalway in the construction ofthe Punktograph (Fig. 74); the buttons are replacedby two metallic rings mounted on ebonite handles(because of their radioscopic transparency) and provision is made to mark the centres ofthese rings—as soon as theyare in their proper position—on the skin of the patient, bymeans of dermic pencils, whichnormally are drawn back andout of the way, bu
. Practical radiography : a handbook for physicians, surgeons, and other users of X-rays . 72. placed on the body, and fromit the point of incision markedon the skin (Fig. 73). The same principle isutilized in a more practicalway in the construction ofthe Punktograph (Fig. 74); the buttons are replacedby two metallic rings mounted on ebonite handles(because of their radioscopic transparency) and provision is made to mark the centres ofthese rings—as soon as theyare in their proper position—on the skin of the patient, bymeans of dermic pencils, whichnormally are drawn back andout of the way, but spring intothe rings on a trigger being re-leased. The mode of applicationis very simple :— With the limb or bodyplaced in the usual way betweenscreen and tube, but so thatthe latter is laterally displaced,the rings are pressed against theanterior and posterior surfaces of the limb and their positionsshifted until the ring shadows as seen on the fluorescentscreen appear in perfect register and contain in their innerspace the foreign body (or if the latter be large, one of its. Fig. 73. Localization 155 margins). At this moment the triggers are released andthus two dots marked on the skin. This process is re-peated with the limb or the tube showing the oppositelateral displacement and a further two dots are thus markedAll that is now required to find the depth, x, of the foreign WM Fig. 74. object, is to measure the distances between the clots onboth anterior and posterior surface (a and b) and the thick-ness (t) of the limb midway between the two sets of dots. Ob By triangulation we obtain X = ——y x t (Fig. 75).It will thus be seen that neither
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