414 



NATURE 



[April i, 1922 



Radiology and Physics J 

 By Dr. G. W. C. Kaye. 



THE appreciation of the physicist by the medical 

 worker in this country is of recent growth, but 

 radiologists, while fully aUve to the enormous part that 

 radiology will play in medicine in the future, are only 

 awakening to the fact that, if radiology is to advance 

 as it should, they will have to correlate it continuously 

 with physics. They may not find such correlation very 

 easy. Not that physicists would look askance at the 

 idea ; the difficulty is that there are so few of them who 

 are interested. The physicist has never been taught 

 to look upon radiology as offering a possible career. 

 Even had he been prepared to risk it, he would not have 

 found educational facilities to put him on his way. 

 There are probably not half a dozen physicists employed 

 in radiology in this country. The Germans discovered, 

 long before the war, that the secret of progress in 

 radiology was to bring the medical man and physicist 

 continually together and let them work side by side. 

 They went further and introduced them both to the 

 manufacturer — but that is another story ! Is the 

 British radiologist in a position to submit techniques, 

 backed up with a wealth of physical and scientific data 

 such as the German has recently given to the world } 

 It is to be hoped so, but the British radiologist is sadly 

 handicapped by not being able to look to* the physicist 



ultra-violet ray and the longest X-ray, but within the 

 last few months it has been discovered that the con- 

 tinuity is complete and that the X-rays follow on and, 

 indeed, overlap the ultra-violet end of the spectrum. 

 The study of this missing group of octaves had invited 

 attention for some time. The grating method proved 

 unavailing for the purpose, the wave-lengths being too 

 small for our artificially ruled gratings and too big for 

 crystal gratings. Further, at either end of the gap the 

 vacuum spectrometer had proved necessary owing to 

 the extremely absorbable nature of the rays. The 

 problem has finally been attacked with success in this 

 country and America by Millikan, Richardson, Hughes, 

 and Kurth who, using indirect photoelectric methods, 

 have traced X-ray spectrum lines of various elements 

 right across the gap and into the already explored 

 ultra-violet. Fig. i shows the positions of some of 

 these lines. 



The following are the wave-lengths in Angstrom 

 units, i.e. lo""^ cm. of the regions of the spectra we have 

 been discussing : — 



12 6 A.U. 



ULTRA - VIOLET 



-4. OCTAVES GAP- 



wm^ 



250 560 1000 1 

 I 2^ I 500 : I$a0 . 

 " 5 55 Z^ 12 I 



Volfc 



AU 



C(L) Al(M) 



AI(L)1 FeOTMbHZnd); 



Bo(K) c(K) o(K); km 



for the discharge of duties which he has neither time 

 nor, possibly, inclination to see to himself. 



Radiology needs men who have a sound knowledge 

 of the physics of radiology and are, furthermore, well 

 grounded in electrical engineering, especially on the 

 high-tension side of the subject. If we could ensure 

 a steady supply of qualified physicists and electro- 

 technicians who knew that in their future work they 

 need not fear that they will not enjoy, both profes- 

 sionally and socially, the full status of their medical 

 colleagues, we could look forward to a desirable all- 

 round improvement in the science and art of British 

 radiology. 



Within the last decade we have learnt that X-rays 

 are identical with light rays in almost every particular, 

 the main difference being that the wave-lengths of the 

 X-rays are much shorter. Until recently, a gap of 

 about 4 octaves existed between the shortest known 



• Abridged from the Mackenzie-Davidson Memorial Lecture delivered 

 on February 17 at the Royal Society of Medicine. 



NO. 2735, VOL. 109] 



It thus appears that we can now claim a knowledge of 

 the existence of over 13 octaves of X-rays or, includ- 

 ing radium y-rays, nearly 16 octaves. As yet the 

 radiologist has only turned about 3 octaves of these to 

 account. 



As is now well known, the parallelism between light 

 rays and X-rays is maintained by the presence of 

 spectrum lines in the X-ray spectra. Just as the 

 spectrum of a hot body normally consists of a con- 

 tinuous spectrum of white light, together with certain 

 spectrum lines the wave-lengths of which are character- 

 istic of the radiating material, so an element emitting 

 X-rays not only gives out " white " radiation, but super- 

 poses its characteristic lines on the general spectrum. 

 The characteristic X-ray spectra are found to be much 

 less complicated than light spectra and are more readily 

 sorted out into groups or series of associated lines. 

 These several series, each of which includes a number of 

 lines, are designated — J, K, L, M — and are broadly 

 differentiated by a progressive increase in the average 



