1916] on Electrical Methods in Surgical Advance 665 



ladies and gentlemen will look through these glasses in the ordinary 

 way, holding them close to the eyes, they will be able to see this for 

 themselves. This effect will be possible, of course, only for those 

 who are able to see with both eyes equally well and have binocular 

 sight. Once the stereoscopic picture is obtained, it becomes exceed- 

 ingly realistic. I would ask those who are able to see these in relief, 

 to turn the glasses round so that the eye which had the red glass in 

 front of it now has the green glass, and vice versa ; under these 

 circumstances, they will find the perspective to be reversed, and while 

 in the first instance the narrow end of the wire cone appeared to be 

 nearer, in this case the larger end appears to be projecting itself 

 towards the observer. With the X-ray tube we carry out precisely 

 the same principles. We take one photograph from one point of 

 view, then displace the tube to a known distance, and take a second 

 photograph. The resulting negatives on casual inspection appear to 

 be identical, but on placing them in the stereoscope, they appear in 

 relief, and the exact position of the bullets can be seen in relation to 

 the other parts of the human body. 



Any practical use which is made of this method, of course, 

 depends upon the ability of the observer to estimate the size and 

 position of the object, and although it is most useful and invaluable 

 in many respects, it is not precise enough for the purpose of exact 

 localization, and in order to arrive at the measurements required 

 with mathematical accuracy we adopt a different system. In the 

 first place, we require to have a frame in which a pair of wires are 

 made to cross each other at right angles ; this is placed in contact 

 with the fluorescent screen, and the small electric light, which in 

 this experiment represents the X-ray tube, is adjusted opposite the 

 point w^here the wires intersect and at right angles to the plane of 

 the screen. If, now, any object be introduced, such as a bone with 

 a bullet in some fixed position in relation with it, a shadow of this 

 object will appear on the screen. On the light being displaced side- 

 ways, to right or left, or upwards or downwards, to a fixed distance — 

 6 cm. being the most convenient because it coincides approximately 

 with the space between the eyes of the observer — a corresponding 

 shift will be seen to take place in the shadow, and the relative 

 positions of the objects will accordingly be altered. The distance 

 between the source of light and the screen having been carefully 

 measured, a tracing may be taken of each of the shadows of the 

 cross wires and the object when the tube was occupying the two 

 positions. This tracing will reveal the amount of displacement, or 

 parallax as it is called, of the shadow of the bullet in relation to 

 the bone. 



The simplest way of interpreting this double tracing is to 

 reconstruct the geometrical conditions under which the shadows were 

 produced. This is most readily done by using a small table with a 

 cross marked on its base, and an upright T-piece above it. Two 



