604 Sir James Mackenzie Davidson [May 5, 



accurately enongb. It is a shadow of the object, and therefore may 

 be very misleading. As an illustration of its possibilities of error, I 

 may throw on the screen a slide showing, first, an ordinary photo- 

 graph of a human skeleton with German bullet situated on the 

 front of the breast bone, in the middle line. That shows the actual 

 situation of the bullet, and next we have the X-ray photograph of 

 this same skeleton with the tube placed centrally. Here it will be 

 observed that the shadow of the bullet appears to be over the spine ; 

 it is still shown in the correct middle line, but no information what- 

 ever can be gathered from this picture as to its depth, and I do not 

 think that anybody would believe, from a glance at this illustration,, 

 that the bullet could be situated on the front of the breast bone. 

 Next we have two X-ray pictures of exactly the same subject, in 

 which, however, the X-ray tube occupied sHghtly different positions. 

 In the one picture the bullet appears to be in the lung, to the right 

 side of the spine, and in the other, for which the tube was shifted to 

 a slightly different position, the bullet appears to be in the opposite 

 lung. I have prepared this example in order to show how fallacious, 

 the single X-ray photograph may be so far as concerns the imparting 

 of correct information with regard to the position of the bullet. In 

 spite of all this, when X-rays are properly used, they can afford data 

 which enable us to ascertain the exact position of a bullet and its- 

 exact size. 



To illustrate the methods now adopted, I propose to use a small 

 electric lamp instead of an X-ray tube ; it is easier to manipulate, and 

 offers quite a serviceable analogy. I have here an opalescent screen 

 to represent the ordinary X-ray screen, and a little electric lamp 

 which will, for the moment, take the place of the X-ray tube, or,, 

 rather, of the point in the X-ray tube from which the rays originate. 

 If I interpose a wire cone between the light and the screen, the 

 shadow of the cone will be observed to give, on the flat, no proper 

 idea of the shape of the object itself. If, however, I displace the 

 lamp to one side, it will be found that the shadows alter their respec- 

 tive positions, and although the second single image may be as little 

 informing as the first, it is upon the effect of this displacement that 

 our precise methods are based. 



In the first place, there is the stereoscopic method. If I have 

 two little electric bulbs situated side by side, one of them being sur- 

 rounded by a green film of gelatine, and the other by a red film of 

 gelatine, it will be observed that each casts a shadow of the object, 

 from slightly different points of view. Accordingly we have a red 

 shadow and a green shadow of the object. If each eye can be made 

 to see its own image, the combined shadows will give the observer an 

 impression of solidity, and he will see the shadows of this wire cone 

 apparently in relief, just as though he were looking at the actual 

 object. In order that this may be demonstrated, red and green 

 spectacles have been handed round among the audience, and if the 



