520 



NA TURE 



[April 22, 1922 



relative intensities of some of the lines of the secondary 

 spectrum alter in a surprising manner, some of the 

 lines being greatly enhanced whilst others become very 

 weak. 



From a theoretical point of view the spectrum of 

 helium is second in importance only to that of hydrogen. 

 The lines of heUum are prominent in the spectrum of 

 the chromosphere of the sun and of many stars, and 

 their relative intensity varies under different conditions 

 of excitation in the laboratory and in different celestial 

 spectra. There are six chief series of lines in the 

 spectrum of helium, three of which are usually referred 

 to as the " helium " and three as the " parhelium " 

 series. The helium series are the stronger in vacuum 

 tubes containing the gas at pressures exceeding a few 

 millimetres, whilst at very low pressures the parhelium 

 series are predominant. Since the chief visible line of 

 the helium series is yellow and that of the parheUum 

 series green, the colour of the discharge is changed 

 from yellow to green when the pressure is reduced. 



There is another spectrum associated with helium 

 which is analogous to the secondary spectrum of 

 hydrogen in that it appears with any considerable inten- 

 sity only when the gas is exceedingly pure. This spec- 

 trum is known as the band spectrum of helium, and its 

 ■occurrence in a gas which is known to be incapable of 

 forming molecules in the chemical sense of the word 

 is very remarkable, in view of the fact that band spectra 

 are generally attributed to molecules. It may perhaps 

 be suspected that there is some temporary association 

 of atoms during the passage of the electric discharge 

 which cannot be referred to as a molecule in the chem- 

 ical sense of the word. Prof. A. Fowler has shown 

 that the arrangement of the heads of the bands in 

 this spectrum resembles that found in series of lines 

 which are due to atoms, though the arrangement of 

 the lines which constitute each band is of the type 

 usually found in band spectra. 



When powerful condensed discharges are passed 

 through helium a spark spectrum is developed. Two 

 series in this spectrum are known as the 4686 and the 

 T Puppis series, and their discovery by Prof. Fowler 

 has led to some of the most important developments 

 •of theoretical spectroscopy. These spark lines of 



helium are found in the nebulae and early type stars, 

 and are attributed to helium atoms which have lost 

 an electron. 



The energy required to produce spark spectra varies 

 widely with the nature of the gas under investigation, 

 and for elements of the same chemical group is, as a 

 rule, smaller the greater the atomic weight of the 

 element. Thus in the case of helium powerful dis- 

 charges are required for the production of the spark 

 spectrum and the lines of the arc series are always 

 bright. In the case of argon a much less intense 

 discharge is required to produce the spark lines, and 

 with very powerful discharges the arc lines disappear 

 almost entirely from the spectrum. In addition to the 

 production of these spark spectra one of the effects of 

 powerful condensed discharges is to alter the relative 

 intensities of the arc lines. Generally speaking, the 

 effect of an increase of energy on a particular series 

 of lines is to enhance relatively the more refrangible 

 members of the series, but the effect varies in degree 

 for different series. Experiments of this kind enable 

 us to imitate to some extent in the laboratory the 

 distribution of intensity amongst the lines which is 

 found in the nebular and stellar spectra. 



It will be seen that whilst many variations in spectra 

 can be referred to different compounds, to molecules, 

 and to uncombined atoms in successive stages of 

 ionisation, there are a number of other changes for 

 which there is at present no obvious theoretical ex- 

 planation. The possibility of some specific influence 

 of one gas on the spectrum of another must now be 

 recognised apart from the formation of chemical com- 

 pounds, which, in the action of helium on the spectrum 

 of hydrogen, for example, appears to be excluded. 

 There is also other evidence, based on a study of the 

 broadening of spectrum lines, of a specific action on 

 neighbouring atoms. We are still awaiting a satis- 

 factory theoretical explanation of phenomena of this 

 kind, though it is now forty years since what is 

 perhaps the first known example, the action of sodium 

 on the absorption spectrum of magnesium vapour, was 

 observed by Prof. Liveing and Sir James Dewar at 

 the Royal Institution. 



Mathematics and Public Opinion. 



TDERHAPS few well-known mathematicians have 

 ->- escaped an experience which would be amusing 

 if it were not so exasperating. Mr. Brown (let us say) 

 is introduced to Prof. Smith, who teaches mathe- 

 matics at a provincial college. After the usual expres- 

 sion of pleasure at the introduction. Brown generally 

 adds " Of course, although I haven't had the pleasure 

 of meeting you before, I know you well by reputation." 

 Then, without so much as pausing to take breath, he 

 proceeds to explain that he was always a duffer in 

 " maths " at school, and that he has now forgotten 

 •everything about the subject they tried to teach him 

 as a boy. Now Brown doesn't act in this way to 

 every celebrity. If introduced to Dr. Lasker, and 

 unaware that he is a distinguished mathematician, 

 he does not seize the first opportunity of telling him 

 that, although he occasionally plays draughts with 

 his wife in the evening, chess was always beyond him, 



NO. 2738, VOL. 109] 



and he could not remember the simplest openings. 

 Still less does he act in this way if his new acquaintance 

 is a sportsman or an epicure. Moreover, in making 

 his lamentable confession. Brown shows no sign of 

 regret or humiliation ; on the contrary, a sort of 

 satisfied look steals over his face, suggesting that he 

 is glad to be free once for all from the study of such 

 a repulsive and useless subject. England is perhaps 

 the only country where such an occurrence is fairly 

 frequent ; and this fact suggests some very unpleasant 

 reflections. 



One thing clear from Brown's attitude is that he 

 evidently fears lest Smith should introduce some 

 mathematical topic during the conversation. Of 

 course this is the thing Smith is most unlikely to do. 

 If this were all, it would be as harmless as the cari- 

 catures of professors and policemen which we see on 

 the stage. But there is a very serious additional 



