April r, 1922] 



NA TURE 



415 



wave-length of each group as we pass from one to 

 another, series J having the shortest wave-length and 

 requiring the highest voltage to excite it. It should 

 be added that all the constituent lines of a group are 

 excited simultaneously at a critical minimum voltage. 



The work on X-ray spectra has thrown great light on 

 the structure of the atom, and, in passing, it may be 

 recalled that present-day theory regards all atoms, of 

 whatever kind, as built up of two kinds of " bricks," 

 and two only — (a) negatively charged electrons, and {b) 

 hydrogen " nuclei," each more than 1800 times as heavy 

 as an electron and carrying a charge equal to that on 

 the electron, but positive in sign. Rutherford's nucleus 

 theory of the atom, now universally accepted, regards 

 an atom as built up of a minute positive nucleus (to 

 which practically the whole mass of the atom is attri- 

 buted) surrounded by a cluster of electrons grouped in 

 rings. The total number of electrons in these rings is 

 equal to the atomic number (N) of the atom in question. 

 The nucleus of the atom is regarded as built up of 

 i^ydrogen nuclei held together by electrons, the former 

 being in excess to just such an extent that the nucleus 

 as a whole contains N positive charges. This serves 

 to counterbalance the N negative charges of the 

 electron rings, the result being an electrically neutral 

 atom. For example, platinum has an atomic number 

 of 78. Its atomic weight determined chemically is 195. 

 Thus, if platinum is a simple element, the platinum 

 atom has a nucleus composed of 195 hydrogen nuclei 

 and 117 electrons, the difference (78) serving to counter- 

 balance the 78 electrons in the rings. The various 

 elements differ only one from another in that they have 

 different nuclear charges, the nucleus determining the 

 mass and radioactive properties, while the number 

 and grouping of the cluster of electrons in the rings 

 control the chemical and spectroscopic properties. 

 For example, the K radiation is supposed to arise from 

 the displacement of an electron in the innermost ring, 

 the L radiation from the next ring, and so on. 



Within the last few years it has been established ex- 

 perimentally that there is a definite boundary to every 

 spectrum of general X-rays on its short wave side. 

 The position of this boundary (or quantum limit) is 

 not affected by the nature of the element emitting the 

 X-rays, but is dependent solely on the maximum volt- 

 age applied to the tube. The relationship is given by 

 the well-known quantum equation of Planck. Sub- 

 stituting the accepted values of the constants, it 

 follows that 



12,400 

 ° ""shortest wave-length in A.U. 



This very simple relation provides us with a scale 

 of quality which, if not perfect, is more exact than any 

 which the radiologist has been in the habit of using. 

 Spectral curves of X-ray intensity are not symmetrical, 

 the shortest waves are the dominating ones. The 

 mean effective wave-length (or " centre of gravity ") 

 of a spectrum of rays approximates to the wave-length 

 of the peak of the curve, i.e. the wave-length of maxi- 

 mum intensity. Now there is some evidence that this 

 wave-length of the peak (X,„) is proportional to the 

 limiting or quantum wave-length (Ao) ; in many cases 

 A,,, proves to be approximately 4/3 times Aq. But in 

 practice it is much easier to measure A„ than A,„, and 



NO. 2735, VOL. 109] 



this fact gives an added importance to the measure- 

 ment of the quantum limit and enables us to identify 

 very fairly the quality of a mixed bundle of X-rays. 

 No doubt something depends on the wave-form of the 

 exciting potential, but the effect of this is probably 

 less important as the voltage is raised. The precision 

 of the method would be enhanced if steps were taken 

 to standardise apparatus and technique, so that all 

 work could be done by the use of, at most, three or 

 four spectra the distinctive features of which, including 

 energy distribution, could be determined and specified. 

 But how is the radiologist going to measure wave- 

 lengths in his operating room ? At present, the easiest 

 plan appears to be by measuring the maximum voltage 

 and using Planck's relation. The voltage can be ob- 

 tained by use of a reliable type of electrostatic volt- 

 meter, or, failing that, by measuring the alternative 

 gap by means of some approved type of spark gap such 

 as the sphere gap. Another and a better plan is to 

 measure the quantum limit by means of a portable 

 direct-reading spectrograph of the type designed in 

 Germany by Seemann (Fig. 2). Incidentally, these 

 direct-reading spectrographs act as very convenient 



Leod Wed^e 



Photographic plate / 



and accurate high-tension voltmeters, which afford a 

 measure of the true maximum voltage effectively 

 operating a tube. 



There are two things that may happen to a beam of 

 X-rays when passing through a material. Part of it 

 may be absorbed, and is therefore wholly transformed 

 into characteristic radiations of the material, the 

 process always being accompanied by the liberation 

 of electrons. The rest of the beam is scattered or dis- 

 persed which, in effect, is equivalent to stating that 

 while the rays are unaltered in quality a considerable 

 proportion of them have their direction altered. 

 Scattering, which finds a close parallel in the dispersion 

 of light by a fog, is more noticeable with light atoms 

 than with heavy. 



We explain these two effects — absorption and scatter- 

 ing — by supposing that absorption is caused by the 

 flicking off by the X-ray of an electron in one of the 

 ring systems in the atom. The outcome is the vibration 

 of the ring systems in question with characteristic 

 periods and the expulsion of an electron from the atom 

 at high speed. If, on the other hand, it happens that 

 the X-ray is incapable of definitely ejecting an en- 

 countered electron but merely jars it, so to speak, then 

 the electron, having absorbed the energy of the X-ray, 

 vibrates not with its own free period but with a forced 

 period which is prescribed by the X-ray and it re-emits 

 its new-found energy in all directions, though chiefly 

 round and about the original direction. With a medium 

 weight or heavy atom the proportion of scattered to 

 absorbed radiation depends upon the wave-length and 



