1902. J on X-Bays and Localisation. 133 



two lines scratched upon it at right angles to each other, and crossing 

 at the middle point. A small T-piece of metal with three notches 

 in it is supported on a vertical rod, and can slide up and down. It 

 is so arranged that the three notches are in a line parallel with one 

 of the diamond scratches, and three centimetres separate the centre 

 notch from the adjacent ones on either side. The tracing from the 

 negatives on the celluloid is placed upon the stage with the crosses 

 superimposed. The metal support, with the notches, is then fixed so 

 as to be the same height from the stage as the anode was from the 

 cross-wires. The two lateral notches now represent the two positions 

 to which the anode of the Crookes' tube was respectively displaced, 

 and as X-rays travel in straight lines without deviation, fine threads 

 can be used to trace their path. Therefore if a thread, attached to a 

 needle, is placed upon the end of the tracing of one of the needles, 

 and a thread through the other notch is placed upon its own shadow 

 at a corresponding point of the needle, the threads will intersect, 

 and the point where they intersect represents the actual position in 

 space of that end of the needle. Now the distance of this point of 

 intersection from the photographic plate is the actual depth of that 

 point below the surface of the patient's skin next the photographic 

 plate. If a perpendicular plane be raised upon each of the hori- 

 zontal wires, and the vertical distance from each taken, we get three 

 measurements which enable us, from the markings of the cross on 

 the patient's hand, to fix a point upon his skin beneath which, at a 

 known depth, the foreign body is certain to be found. In geometrical 

 language, we get three co-ordinates of the point in relation to three 

 planes at right angles to each other, and as the relation of the patient's 

 body to these planes is also known, it is a simple matter to decide 

 the actual position of the needle or other foreign body ; more than 

 that, the size and shape of the foreign body can be ascertained. But 

 these two photographs, while enabling this information to be derived 

 from them of ordinary triangulation, are really stereoscopic, and 

 when viewed in a Wheatstone's stereoscope by an observer possessing 

 binocular vision, the eyes will triangulate the negatives, as it were. 

 The negatives should give a combined image in perfect stereoscopic 

 relief, so that by this method not only can actual measurements ana 

 data be given to the surgeon, but a view of the parts be also aflorded. 

 As an indication of how reliable this method is, I will mention that 

 I have applied it in detecting very minute particles in the eyeball 

 and orbit in a great number of cases, and this proves of signal service 

 in eye surgery. I have detected and located in the eyeball particles 

 of glass, steel, brass, iron, copper, silver and lead. The method for 

 localisation is based upon the same principles. 



In warfare as well as in civil life the X-rays when properly used 

 are of invaluable service, and I consider that the stereoscopic method 

 is of the highest value in giving the maximum and accurate informa- 

 tion that could possibly be afforded by X-rays. Several examples of 

 stereoscopic skiagrams are exhibited in the Library, and I am glad to 



