4 Transactions of the Society. 



structed, emits radiations of various wave-lengths. If the waves are 

 exceedingly short, the tube is referred to as a " hard " one, the rays 

 emitted being termed " hard " rays. On the other hand, if the tube 

 is not so highly exhausted, and emits rays of longer wave-length, the 

 tube is then referred to as " soft " and the rays emitted are called 

 " soft " X-rays. The difference between these two states so far as 

 penetration is concerned is very marked. Hard X-rays will pene- 

 trate nearly all substances even if their thickness is considerable, 

 but with soft X-rays penetration is not nearly so great ; and for all 

 small objects, which are in any sense microscopic, their use is 

 indicated. The tube I have used is provided with a lithium glass 

 window so that the soft X-rays are freely transmitted. Ordinary 

 glass obstructs the softest X-rays very seriously. 



If it were possible to produce X-rays of any given wave-length, 

 in the same sense that we can obtain monochromatic light for 

 ordinary work, it would help us considerably. There is much 

 reason to think that this will be practicable before long. X-ray 

 tubes as ordinarily constructed emit rays of varying wave-length, 

 so that while you can get a " soft " tube emitting a preponderance 

 of soft rays, yet there is a certain admixture of hard rays with it. 

 .Another point that influences the particular quality of the rays 

 emitted, is the material of which the anti-cathode is made. As you 

 are probably aware, X-rays are generated in an exhausted tube as 

 the result of the impact of cathode rays on a metal surface, this 

 metal surface then becoming the source of X-rays. By varying 

 the material of which the anti-cathode is made, changes are pro- 

 duced in the character or wave-length of the X-rays emitted. 



The method of producing the few radiographs I am showing you 

 this evening is very simple. An X-ray tube is enclosed in a 

 lead-covered box so that the rays cannot pass out except in the 

 desired direction. There is a small aperture in the position oc- 

 cupied by the lithium glass window in the tube, and this is covered 

 with a lead diaphragm which has a small central circular hole. 

 There are two or more of these diaphragms one above the other, 

 with a certain distance between, so that at a distance of 15-20 cm. 

 from the tube a parallel beam of X-rays of very small cross section 

 is transmitted. The object is then laid in a light-tight box in the 

 path of the beam, the rays passing through the object and imping- 

 ing on to a photographic plate which is placed in actual contact 

 with the object. The photograph then results as in ordinary X-ray 

 work. 



The resulting enlargement is purely a photographic process. I 

 found very early that the results were limited, at least in part, by 

 the size of grain in the photographic plate ; and any ordinary X-ray 

 plates that 1 tried were not satisfactory from this point of view, 

 although good enough for ordinary X-ray work. In the course of 

 some ultra-violet light experiments which I had previously con- 



