December, 1910. 



KNOWLEDGE. 



495 



He also found 



straight lines from the 



\\ 



\ 



£ 



elements composing the substance 



that the radiation came in 



point of their origin, and that thev could not be 



reflected or refracted, and were not in 



any wav influenced by a magnet. It 



was no\s' evident to him that he was 



in the presence of radiations hitherto 



unknown, and he forthwith set about 



an exhaustive study of their phvsical 



properties, so as to determine their exact 



nature, and so perfectly did he do this, 



that jhis original monograph has never 



been seriously added to up to the 



present time. 



Having shown how an\- inequalities 

 in density in any given object would be 

 registered on a photographic plate, he 

 at once recognised the immense import- 

 ance of the discovery to surgery, and 

 made his communication to the 

 Physico-Medical Societv of W'urzburg 

 in December, 1895. 



To describe in detail the various steps 

 in the development of all the apparatus The original Crookes's Tube, 

 used in radiologv would, 

 and does. 



\ 



j5v tAe courtesy of Sir Witliain 

 Crookcs^O.M., F.R.S. 



Figure 1. 



good-sized 



By the courtesy 

 The X-Ra' 



fill man\- a 

 volume : as 

 witness the excellent 

 books written by several 

 authors both here and 

 abroad. \\'e may, how- 

 ever, trace the evolution 

 of the more important 

 parts of the modern X-ray 

 outfit without going 

 beyond the space usually 

 allotted to such an article 

 as this. 



In view of the fact that 

 the whole science of radiologv centres 

 around the X-ra\- tube, and that everv- 

 thing else is subsidiarv to it, we ma\ 

 very fitly consider this first. The original 

 experiments were made with tubes of the 

 well-known " pear " shape originated bv 

 Crookes. In these the kathode stream 

 was made to impinge upon some part of 

 the wall of the tube. (X-ra\s are pro- 

 duced when a stream of kathode rays are 

 suddenly arrested bv impact against a 

 solid body.) The area of impact was 

 always large — sometimes the whole of 

 the large end of the tube showed the 

 characteristic green fluorescence, show- 

 ing that the X-rays were beiu; 

 off from its whole area, 

 conditions the imaijes of 



Under such 

 a fluorescent 

 screen or a photographic plate were much 

 blurred and generally indistinct. This 

 was, of course, a very great disadvantage, 

 and had no other form been available, it 

 is safe to say that the science of radiology 



By t/ie courtesy o/ Messrs. 

 Xcuiton <S^ Company. 



Figure 3. 

 A modern X-Rav Tube. 



would have made ver}' little progress. The first 

 and the most important single contribution to the 

 science was made by Professor Herbert Jackson, 

 of King's College, London. He made 

 a tube with a concave kathode, and 

 ^ mounted at or near the focal point, a flat 



disc of platinum w hich also acted as the 

 anode of the system. The result was 

 that all the X-rays came from this 

 point, and gave sharp shadows with clear 

 outlines as well as details of minute 

 structure. Since Jackson made his 

 original tube thousands have been made 

 bv manv makers, but his general plan of 

 construction has never been altered. In 

 all tubes working under normal con- 

 ditions, the tendency is for the vacuum 

 to slowly increase. The generally 

 accepted explanation of this is that the 

 electrons become embedded in the walls 

 of the tube, and so are put out of action. 

 It is even believed that some are driven 

 right through the glass. A certain 

 number of those embedded can be 

 restored to the body of 

 the tube by heating the 

 glass, and this was the 

 onlv method of regen- 

 erating a tube in the 

 earliest days of X-rays. 

 The need for regenera- 

 tion w as owing to the fact 

 that with the increase of 

 vacuum the tube became 

 so resistant that the 

 electric current passed 

 around the outside in 

 preference to going 

 through. Also the char- 

 acter of the radiation altered, so that a 

 "hard" tube, as it is called, gave very 

 poor images on the photographic plate, 

 although the screen appearances were 

 fairlv brilliant : the reason being that the 

 ravs from a hard tube are less active in 

 affecting the sensitive salts of silver, and 

 hence it does not necessarily follow that 

 a tube that gives a satisfactory image of 

 the screen will give a good account of 

 itself when used for registering the 

 image on a sensitive plate. 



^Ianv devices have been brought out 

 from time to time for regeneration of 

 the vacuum in a convenient manner, 

 and each has certain advantages. At the 

 present time there seems to be an almost 

 universal tendency among tube makers 

 to employ some porous material such as 

 asbestos, spun glass, leaves of mica, and 

 so on. The material is enclosed in a 

 small accessory bulb, the cavity of which 

 communicates with that of the X-ra\- 



.. .,. ..'ton ^ Company. 



tiGURE 1. 



Tube as improved by Professor Herbert Jackson 



