. Radiography and radio-therapeutics . This is done by means of thecross-thread locaHser (Mackenzie Davidson). The apparatus consists of anadjustable horizontal bar, which is marked with a milhmetre scale startingfrom a central zero, and is notched to correspond on its upper edge ; aplate-glass stage marked with two lines at right angles to each other, the pointof intersection lying exactly beneath zero on the horizontal bar. Beneaththe stage is a hinged reflecting mirror. The developed negative or tracingsof the two plates on a celluloid sheet is placed film upwards on the glassstage, and the
. Radiography and radio-therapeutics . This is done by means of thecross-thread locaHser (Mackenzie Davidson). The apparatus consists of anadjustable horizontal bar, which is marked with a milhmetre scale startingfrom a central zero, and is notched to correspond on its upper edge ; aplate-glass stage marked with two lines at right angles to each other, the pointof intersection lying exactly beneath zero on the horizontal bar. Beneaththe stage is a hinged reflecting mirror. The developed negative or tracingsof the two plates on a celluloid sheet is placed film upwards on the glassstage, and the shadow of the wires made to correspond with the cross markon the stage. The bar is next raised or lowered so as to bring the zero of thescale to the same distance from the scale as that of the centre of the anodefrom the sensitive plate when the exposures were made. Two fine silk threadsare next passed over the horizontal scale bar. Each thread has a weight atone end to keep it taut, and is fixed in a notch on the scale corresponding. MACKENZIE DAVIDSON METHOD 163 to the distance of the anode from zero during the original exposure,other end is threaded The
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