February 13, 191 3] 



NATURE 



647 



In this connection it is interesting to note 

 that X3 does not appear to occur to any 

 appreciable extent in the atmosphere. Some- 

 times when suffering from the difficulty of clear- 

 ing out these gases I have been goaded into speculat- 

 ing whether they do not represent the partially 

 abortive attempts of ordinary metals to imitate the 

 behaviour of radio-active substance ; but whereas in 

 these substances the a particles and the like are 

 emitted with such velocity that they get clear away 

 from the atom, in ordinary metals they have not 

 sufificient energy to get clear, but cling to the outer 

 parts of the atom, and have to be helped by the 

 kathode rays to escape. 



I would like to direct attention to the analogy 

 between the effects just described and an everyday 

 experience with discharge tubes — I mean tlie diffi- 

 culty of getting these tubes free from hydrogen when 

 the test is made by a sensitive method lilie that of 

 the positive rays. Though you may heat the glass of 

 the tube to melting point, may dry the gases by 

 liquid air or cooled charcoal, and free the gases you 

 let into the tube as carefully as you will from 

 hydrogen, you will still get the hydrogen lines by the 

 positive-ray method, even when the bulb has 

 been running several hours a day for nearly 

 a year. The only exception is when oxygen 

 is kept continuously running through the tube, 

 and this, I think, is due, not to lack of libera- 

 tion of hvdrogen, but to the oxygen combining with 

 the small quantity of hydrogen liberated, just as it 

 combines with the mercury vapour and causes the 

 disappearance of the mercurv lines. I think this pro- 

 duction of hydrogen in the tube is quite analogous to 

 the production of X,, of helium, and of neon. I have 

 been greatly assisted in the experiments I have 

 described by Mr. F. W. Aston, Trinity College, and 

 Mr. E. Everett. J. J. Thomson. 



Februarv S. 



The Water-surface "Halo." 



The "halo " which a happy memory of eighty years 

 enables the Rev. O. Fisher to recall in N.wure of 

 February 6 was probably one to which the explana- 

 tion offered by Dr. Franklin Parsons does not apply. 



There is a very striking phenomenon of separate 

 rays or shafts of light converging on the shadow of 

 the observer's head when this shadow is thrown on 

 water. The phenomenon requires for its production 

 certain conditions : — (i) A bright sun, high in a clear 

 sky. For this reason in these latitudes the appearance 

 is best seen about midday in summer. In winter it is 

 scarcely noticeable. (2) The water must not be quite 

 clear ; on the other hand it must not be very turbid. 

 (3) The surface must not be smooth, but may be 

 fairly briskly agitated, but again not too briskly. (4) 

 The water should be deep. 



If any one of these conditions is absent the pheno- 

 menon is not seen, or is only imperfectlv seen, as I 

 was able to satisfy myself about twenty-five years 

 ago by observations made, day after day, on the lake 

 of Ullswater, where a stream discharged the muddy 

 water of a mine far into the lake, and thus provided 

 one of the necessary' factors of variation. The neces- 

 sity of these conditions, when once discovered, makes 

 the explanation easy. The irregular convexities of the 

 ruffled surface acting as condensing lenses separate 

 the light penetrating the water into converging shafts. 

 ."Mong certain lengths of each or many of these shafts 

 a sufficient condensation of light takes place to render 

 them visible by means of the additional illumination 

 of the slight turbidity. Thus the water is filled with 

 luminous parallel shafts of varying lengths, which, 



NO. 2259, vol: 90] 



seen in perspec:ive, have their vanishing point in the 

 shadow of the observer's head. I remember that it 

 was long before I realised that the rays were below 

 and not on the surface. When the observer's head 

 is not many feet above the water the rays may be 

 traced to great distances — 50 or 60 degrees — from 

 the shadow of the head. 



The phenomenon, though often very brilliant, is 

 often unnoticed, even by good observers — I think 

 because it requires a certain comprehensive glance, 

 no doubt in the first instance accidental, to recognise 

 that the widely separated broken radiations belong to 

 a single convergent system. But when this system 

 has once been realised it becomes hauntingly present, 

 and one glimpses portions of it at every glance at the 

 water, even though the shadow of the head is cut 

 off from the surface. A. M. Worthington. 



Exmouth, February 9. 



An X-Ray Fringe System. 



By allowing a diverging pencil of Rontgen radiation 

 to fall at nearly grazing incidence on one of the sets 

 of cleavage planes of a crystal of rock-salt, and 

 observing the intensity of the reflected pencil by a 

 photographic plate, we find a series of well-marked 

 and equal-spaced maxima in positions corresponding 

 to equal increments of cos 6, where S is the angle of 

 incidence of radiation on the cleavage planes. In the 

 directly transmitted beam there is no indication of 

 variation of intensity with angle of incidence We 

 thus have what appears to be a series of X-ray spectra 

 of different orders, due to agreement in phase of 

 waves from successive layers of molecules. Calculat- 

 ing on this assumption we get a wave-length of the 

 order of magnitude in agreement with that calculated 

 from the velocity of ejection of electrons by a sub- 

 stance exposed to this particular radiation — that is, 

 assuming the results of the experiments of A. L. 

 Hughes and others on ultra-violet light are equally 

 applicable to Rontgen radiation. While only few ex- 

 periments have yet been made on which to base any 

 interpretation, this is in agreement with what we 

 have already observed. Of the experimental results 

 there is no doubt, and we cannot at present suggest 

 any probable explanation except the very obvious one 

 of interference. Further experiments are in progress. 



C. G. Barkla. 



G. H. Martyn. 

 King's College, London. 

 February 11. 



Atmospheric Potential. 



In Nature of December 12, 1912 (p. 411), Dr. 

 George C. Simpson directs attention to several out- 

 standing problems in atmospheric electricity. He 

 says, inter alia : " Everywhere it has been found that 

 the air is a conductor, and that the potential gradient 

 is practically the same." It is not the object here to 

 consider these statements, however questionable. 



The potential gradient of the atmosphere is the 

 difference of electric potential between two points in 

 the same vertical one metre apart ; which, for the 

 first few kilometres above the earth's surface, is about 

 100 volts. 



Now one problem which Dr. Simpson does not men- 

 tion is the absence of current from the upper regions 

 of the atmosphere to the lower corresponding to this 

 difference of potential between them. It is a funda- 

 mental law of electricity that an electric current will 

 flow in a conductor from a high potential to a lower 

 one. 



A conductor projecting vertically from the earth's 



