December. 1910. 



KNOWLEDGE. 



499 



automatically reverses the inverse waves. The result 

 is a unidirectional, pulsating current that is entireh' 

 free from reverse waves, and the po^\er of the 

 apparatus is such that instantaneous radiography is 

 readilv carried out with it. 



The drawbacks of this tvpe of generator are the 

 high cost, bulkiness. and noise, among others : but 

 where great power is required it stands easih' first. 

 The greatlv improved coils referred to above con- 

 stitute the answer of the coil makers to the advent 

 of the " high tension rectifier." as these machines 

 are called. For a time these rectifiers had things 

 all their ow n wa\-, but this is no longer the case, and 

 in the writer's opinion it is an instrument of the 

 induction coil t\"pe that w ill ultimateh' prevail, that 

 is, so long as our source of X-ra\s continues to be 

 from a vacuum tube. The reason for this is on 

 account of the peculiar character of the current given 

 out. M the moment of interruption the current 

 rises verv suddenh^ to an extremeh' high tension, 

 considering the output of the coil as a transformer 

 of energv, and these impulses are eminentlv suitable 

 for starting the electrons in action, as well as keeping 

 them going, even when using a very small current. 

 If we were to trace out the voltage of the coil current 

 in the form of a curve, it would represent a mountain 

 range of \"erv high and steep peaks, while that from 

 the rectifier is more like a series of low hills with 

 broad bases and rounded tops. The latter type of 

 current is not so suitable for exciting X-rav tubes, 

 and at times the\- work very unevenly, especially 

 with a small amount of energv. 



Early in the days of radiolog\' it was observed 

 that the intervention of a diaphragm increased the 

 definition and sharpness of the image, in the same 

 way as in the photographic lens : then it was found 

 that if instead of using a hole in a metallic disc the 

 rays were made to pass through a metal tube a still 

 further gain in sharpness was attained. So important 

 is this, that the use of the tubular diaphragm 

 is essential if the best radiographs are to be secured. 



One of the most difficult matters to deal with is 

 that of movement. Some patients have the faculty 

 of keeping still for comparatively long periods of 

 time. V'erv manv others have not : the movements 

 may be voluntarv, but are frequentlv involuntary 

 from nervousness, or tremors that are not under 



control. .\11 the organs in the thoracic, abdominal, 

 and pelvic cavities move to a greater or lesser extent 

 with the movements of respiration, and this is an 

 important factor in assisting the performance of 

 their normal functions. Some of these movements 

 are easih" restrained, others are not, and many 

 devices have been brought forward for this purpose ; 

 but the most generally useful of all is that in which 

 the tubular diaphragm is mounted in a framework 

 that passes across the patient, and is so arranged that 

 it can be low ered down and made to press firml\- upon 

 the part it is desired to examine. Movement can in 

 most cases be perfectly restrained, and the improve- 

 ment in the quality of the resulting image is, 

 compared with that taken without an\- diaphragm, 

 quite startling. One of its greatest uses is in 

 the examination of the kidne\' for calculus (see 

 Figure 5). This organ under normal conditions 

 moves about half-an-inch with each respiration, 

 and if this is not restrained no clear image can 

 be secured. With the compressor diaphragm this 

 is easily done, the outline of the kidney is sharply 

 defined, and its size, shape and position are 

 definiteh- registered. As might be imagined the 

 difficult\- increases with the stoutness of the 

 subject, but this is not nearly so important as 

 it was, and a good result can be secured in nearly 

 all cases, with a little care and trouble. 



The use of this device necessitates the X-ray tube 

 being above the patient, and it is not easy to make 

 a preliminarv inspection with the fluorescent screen 

 which, in many instances, is all that is required. 

 Where manv have to be examined, as in the out- 

 patient department of a hospital, it is a great 

 convenience to have a couch with a canvas top upon 

 w hich the patient is laid. The X-ray tube is fixed in 

 a ray-proof box with an opening at the top through 

 which the ra\'s emerge, and this box can be moved 

 in both directions so as to bring it under any desired 

 part. The canvas being transparent to the ra\-s we 

 can. with theaid of the screen, look through the patient 

 and frequentlv see all that is wrong ; if there is any 

 doubt, an X-ray plate is laid on the place and steadied 

 with sandbags while the exposure is made. This is 

 a convenient method for dealing with a number of 

 cases, and the results are good enough for ordinary- 

 conditions. 



THE SELBORXE SOCIETY .AND PLANT PROTECTION. 



The (juestion of Plant Protection has always been 

 one of the objects of the Selborne Society, and for 

 more than twentv vears it has used its influence to 

 prevent the wanton destruction even of hedgerow- 

 blossoms, and the rooting up of rare plants and 

 ferns. The progress of modern civilization and the 

 putting up of buildings is now- making the question 

 of Plant Protection a serious one, and there are other 

 aspects of it which have to be considered. The more 

 lowly plants are in danger of extermination, and 

 it is most important, quite apart from the preserva- 

 tion of rarities, that tracts of land which are covered 

 with common, but none the less interesting, plant 



communities should be preserved in their natural 

 condition. 



At a recent meeting of the Selborne Society a 

 section was constituted to consider the best way of 

 dealing with the problem. Dr. A. B. Rendle, 

 M.A.. F.R.S., Keeper of I>otan}- in the British 

 Museum (Natural History), has accepted the Chair- 

 manship of the Section, and Mr. A. R. Horwood of 

 Leicester Museum, has been appointed Recorder. 



.All those who are interested in the subject of 

 Plant Protection, are invited to communicate with 

 the Honorary Secretary of the Selborne Society, 

 at 42, Bloomsbury Square, London, W.C. 



