132 Mr. James Mackenzie Davidson [Ai)ril 25, 



Localisation. 



I must now leave the physical side, and proceed at once to 

 tell you something as to the surgical application of these rays in 

 locating bullets, etc., in the human body. From what we have seen 

 this evening, you will readily understand that an X-ray photograph 

 is simply a shadow of the object interposed, and the appearance of 

 a single photograph, however realistic, gives no reliable or accu- 

 rate guide as to the actual relative position of the parts. An X-ray 

 photograph is simply a central projection. It soon became necessary, 

 when the use of X-rays became more general in surgical practice, 

 that some means should be adopted to give reliable data for the posi- 

 tion of bullets, needles, etc., in the body. Various plans have been 

 suggested, but the one which I shall describe to-night is the one 

 with which I am most familiar, and one which gives results as accu- 

 rate as possible. The method may be sliortly described as follows : — 

 The anode of the Crookes' tube is placed vertically above a point 

 where two stretched wires intersect each other at right angles. The 

 vertical distance is measured and noted dowu. The tube is attached 

 to a holder which slides along a horizontal bar, and this bar is care- 

 fully arranged parallel to one of the wires. The tube is then dis- 

 placed 3 centimetres to one side of the zero, a scale on the horizontal 

 bar enabling this to be done correctly. The part of the patient to be 

 photographed is now placed upon the cross wires, which are usually 

 stretched over a space covered by stretched calf-skin. The wires may 

 be brushed over with some aniline dye, so that the mark of the cross 

 wires may be left upon the patient's skin. The photographic plate 

 is now placed beneath the cross wires, and pressed tightly against 

 the parchment. One exposure is made. The photographic plate is 

 then removed and a fresh one put in its place ; the tube is then 

 brought back to zero and moved 3 centimetres the other side, and a 

 second photograph is taken. These two photographs will show a 

 parallactic displacement, as they have been taken from two points 

 6 centimetres apart, approximately the distance between our two 

 eyes ; they will not be exactly alike, and if, for example, we were 

 locating a needle in the hand, the different position of the needle in 

 each photograph in relation to the cross wires can be at once shown 

 by taking a tracing from each negative, with a sheet of celluloid with 

 a cross marked upon it. The celluloid is put upon the photographic 

 film, and the cross upon it is brought into register, that is, super- 

 imposed over the shadow of the crossed wires in the negative. A 

 tracing is then made of the needle, and the same is done with the 

 other negative, and it will then be found that the needle images 

 occupy different positions. To interpret this I use an apparatus 

 called a cross-thread localiser, which enables a graphic reconstruction 

 of the conditions in which these negatives were taken. 



The cross-thread localiser consists of a large sheet of glass with 



