. The Röntgen rays in medical work . chslides up and down upon tAvo vertical brass rods. On this barthere is a millimetre scale with a small notch at each millimetremark. The o° is at the middle point of this bar. On the glassplate there are two lines cut with a diamond at right angles to eachother (just like the cross-wires on the vulcanite plate), and this glassplate, which is movable and can be clamped, is so placed that; thepoint Avhere the lines cross is vertically beneath the o° on the scaleabove, and the line on the glass running right and left (Avhen facingthe scale) is placed parallel
. The Röntgen rays in medical work . chslides up and down upon tAvo vertical brass rods. On this barthere is a millimetre scale with a small notch at each millimetremark. The o° is at the middle point of this bar. On the glassplate there are two lines cut with a diamond at right angles to eachother (just like the cross-wires on the vulcanite plate), and this glassplate, which is movable and can be clamped, is so placed that; thepoint Avhere the lines cross is vertically beneath the o° on the scaleabove, and the line on the glass running right and left (Avhen facingthe scale) is placed parallel to the edge of the scale. In otherwords, they are in the same vertical plane. The scale is noAV raised 112 THE RONTGEN RA TS IN MEDICAL WORK or lowered so that the zero mark on it stands at precisely the sameheight vertically above the negative as the anode of the Crookestube was from the photographic plate or film when the negativewas being produced. The whole apparatus stands on screws, so thatit can be levelled easily. - ;~~ ~. Fig. 60.—Mackenzie Davidsons Choss-Thkead Localized The negative is placed upon the horizontal stage and adjusted sothat the white lines produced by the cross-wires are made to coin-cide with the crossed lines on the glass plate, care being taken thatthe marked quadrant is in the same relative position as it occupiedwhen the photograph was being taken. As a routine practice I THE ELECTRICAL APPARATUS 113 always mark the right upper quadrant as I stand facing the scale ofthe bar. It is important to remember that if the negative is placed on thestage with the gelatine surface downwards, the relations of the partsare reversed, and it should be so placed that the marked quadrant,which would be on the. right and upper corner if the gelatine sur-face was upwards, should lie on the left upper corner. However,as the patients skin has the corresponding quadrant marked uponit, there should be no confusion. Matters being thus arranged, it will be seen that
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