. Radiography and radio-therapeutics . the shadows thrown by the sUding exact locahsation the Mackenzie Davidson method should be employed in Mackenzie Davidson Localisation.—Thecentral ray emitted from an X-ray tube has to be definitely found. To dothis it is necessary to have an arrangement for determining the central ray.^The tube is accurately fixed in the box beneath the table ; the latter movesin two directions on trolley wheels. The central ray is located by means ofcross wires, or by a plumb-line running on pulleys andmoving with the tube, sothat whatever the
. Radiography and radio-therapeutics . the shadows thrown by the sUding exact locahsation the Mackenzie Davidson method should be employed in Mackenzie Davidson Localisation.—Thecentral ray emitted from an X-ray tube has to be definitely found. To dothis it is necessary to have an arrangement for determining the central ray.^The tube is accurately fixed in the box beneath the table ; the latter movesin two directions on trolley wheels. The central ray is located by means ofcross wires, or by a plumb-line running on pulleys andmoving with the tube, sothat whatever the positionof the tube the j)lumb-linealways indicates the posi-tion of the focus of raysupon the anti - distance of the anodefrom the top of the couchis constant, and should berecorded on a convenientplace on the couch ; thedistance of the plate fromthe top of the couch mustalso be taken into con-sideration. The two addedtogether give the distanceof the anode from the working with thetube above the patient, the. A B, Second position of tube. two positions of the anode s, Skuii or limb D c Fig. 132.—Diagram showing Mackenzie Davidson method.(After Walsh.) First position of tube. D, Shadow thrown on plate by tube ill A , Shadow thrown on plate bytube in B position. are secured by mo^ang the ^ ^ ^°t^,,^fj- , ,^ ,, , ,- - . *=> Dotted lines represent the paths of the rays. tube along a horizontal barwhich is marked w4th a milhmetre scale, running both ways from acentral point at zero. The sensitive plate is placed underneath the patient,and protected in the usual way by black paper, or it may be placed in alight-tight cassette. Two wires are laid at right angles to each other on thephotographic envelope, and so placed that one of them runs in the samedirection as the horizontal bar which carries the tube above, and their pointof intersection Hes beneath zero on the scale. The cross wives may befastened to a thin board or sheet of vulcanite, a
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