666 Sir James Mackenzie Davidson [May 5, 



threads are suspended from the T-piece 6 cm. apart, and the hori- 

 zontal portion of the T-piece is adjusted to be exactly the same 

 distance from the table as the Hght was from the screen or photo- 

 graphic plate in the first instance. As the rays travel in straight 

 paths, the threads can be used to trace the direction of the beam, 

 and if the tracing be so placed on this little table that the centre of 

 the cross-shadow is vertically below one of the little slots from which 

 the thread depends, the thread (which is passed through a weighted 

 needle) can be brought down upon a definite part of the tracing of 

 the bullet shadow, and will then indicate the path of the ray which 

 caused the shadow. It must be remembered, and allowed for, that 

 the shadow to the left was produced when the source of the rays was 

 in the position to the right, and vice versa. If, now, the second 

 thread be brought down to the corresponding point on the tracing of 

 the other shadow, it will represent the path of the ray when the 

 source of light was in the second position, and it follows that the 

 two threads must cross each other in space. This point of inter- 

 section represents exactly the position of the l)ullet in relation to the 

 body of the patient, and its distance from the table is equal to the 

 actual depth of the bullet from the cross wires when the original 

 observation was made. The distance can be accurately measured 

 with a pair of compasses, and the position of the cross wires being 

 already marked on the patient's skin, it is a simple matter to give 

 the measurements which should guide the surgeon immediately to 

 the detection of the foreign body. 



If more rapid procedures are demanded, as they may well be by 

 the exigencies of the present time, these can equally well be carried 

 out on exactly the same principle of similar triangles, the measurement 

 being done actually ona small fluorescent screen. I have here a 

 hand fluoroscope and a device consisting of scale, cross wires, and 

 sliding piece, calibrated so as to enable one by the simplest adjust- 

 ment to determine the depth of a piece of metal below a marked 

 point on the skin by noting the displacement of the shadow on tiie 

 illuminated screen when the tube is moved to a given distance. 



Having ascertained the position of the bullet in the body, we 

 have at hand, fortunately, other electrical aids for the surgeon when 

 he comes to deal with the problem of its extraction. One of the 

 most useful of these adjuncts is the telephone probe as it has been 

 called, or, as I prefer to call it, the telephone attachment. It consists 

 of a telephone to one terminal of which may be attached the surgeon's 

 steel, silver, or nickel-plated instrument, while a carbon plate is 

 attached to the other terminal, which is applied to the patient's skin, 

 moistened with salt-water to render the skin conducting. When any 

 of the instruments come into contact with embedded metals, such as 

 lead, nickel, copper, iron, or the iron alloys used in shell manufacture, 

 a loud click is elicited, and if a rubbing contact be made with the 

 exploring instrument, the click becomes a sharp rattle. The telephone 



