1916] on Electrical Methods in Surgical Advance 663 



be any one of several different metals. Fortunately, we have now 

 at command various methods, of which I shall give you some 

 account this afternoon, for detecting the presence of the embedded 

 metal, and also for determining its exact shape, size, position and 

 the angle at which it is lying, without any additional pain to 

 the patient. 



The discovery of X-rays has revolutionized surgical diagnosis. I 

 had the honour some years ago of giving a discourse within these 

 walls on the subject of X-rays, but many advances have been made 

 since then, and I will endeavour to describe and illustrate the methods 

 now adopted. X-rays are produced in a vacuum tube such as I show 

 you, and proceed from a piece of metal fixed in the centre of the tube 

 and called the anode or anti-cathode, or, more popularly, the target. 

 Kecently a great improvement in X-ray tube construction has been 

 introduced by Mr. W. D. Coolidge, of Schenectady, U.S.A. His 

 tube, which I have the honour also to show you, has some marked 

 advantages, and very heavy currents can be passed through it, an 

 intense X-ray output being the result. When the electrical current 

 is passed through an X-ray tube, cathode rays — streams of electrically 

 charged particles — are generated at the cup-shaped terminal which is 

 called the cathode, and these, impinging with great velocity upon the 

 target opposite, cause the generation of X-rays at the point of impact. 

 X-rays are simply invisible light — as we now know from the re- 

 searches of Professor W. H. Bragg — of extraordinarily short wave- 

 leno^th, and owing to this shortness of wave the rays are able to 

 penetrate matter which is opaque to ordinary light. If the human 

 body be interposed between the X-ray tube and the photographic 

 plate, and the tube be excited by an electric current, the plate will 

 show on development a shadow-image, revealing the inner structure 

 of the tissues through which the rays have passed. The bones being 

 denser than the flesh, these will appear white in comparison, and 

 should any metal be present which is denser than either, it will be 

 rendered still whiter in the negative. The opacity of various kinds 

 of substances to X-rays follows the order of their atomic density ; 

 thus matter composed of light atoms such as flesh, skin, hair and 

 clothes, allows X-rays to pass through very readily, whereas all 

 metals, except the lightest ones, such as aluminium, are extremely 

 opaque. Bone, seeing that it contains phosphorus and lime, stops 

 the passage of the X-rays much more readily than flesh, which is 

 composed of the lighter 'atoms, namely, carbon, hydrogen, nitrogen, 

 and oxygen. 



The shadow picture which is produced by the impingement of 

 the X-rays upon a photographic plate or a fluorescent screen may, 

 therefore, furnish a good deal of information as to the relative 

 densities of any interposed materials; but however realistic this image 

 may appear, it should always be remembered that it is not like an 

 ordinary photograph from which the relative positions can be inferred 



