September io, 1903] 



NATURE 



449 



for example, was doubtless made up of particles subject to a 

 much more vigorous solar repulsion than those formed into 

 th? shorter curved emanation issuing from it nearly in the 

 same direction. In the comet of 1811 he calculated that 

 the particles expelled from the head travelled to the remote 

 extremity of the tail in eleven minutes, indicating by this 

 enormous rapidity of movement (comparable to that of the 

 transmission of light) the action of a force much more 

 powerful than the opposing one of gravity. The not un- 

 common phenomena of multiple envelopes, on the other 

 hand, he explained, are due to the varying amounts of 

 repulsion exercised by the nucleus itself on the different 

 kinds of matter developed from it." 



It is impossible not to be struck by the similarity both of 

 phenomenon described and of language used in this para- 

 graph and in almost any of (he papers on radium. I know 

 this mere superficial similarity is worth very little, if any- 

 thing ; but for centuries the sky has shown us a pheno- 

 menon still not entirely understood, and the inability to 

 remove all difficulty by the aid of radium or similar material 

 is no reason for dismissing the idea of connection without 

 further thought. 



The comet's tail is still a mystery. Let me take the most 

 recent explanation, which was set' forth only three months 

 ago in the Astrophysical Journal in the United States. 

 Those admirable experimentalists Nichols and Hull have 

 for some years been investigating the back pressure exerted 

 by the action of light upon bodies on which it falls. In 

 this they have followed the Russian physicist Lebedew, but 

 in minuteness and delicacy of measurement, and in their 

 successful elimination of disturbances, their results are un- 

 equalled. It is sufficient to say that, difficult and minute 

 as the experiment is, their success is such that the dis- 

 crepancy between the calculated force and that which thev 

 have found is under i per cent. Perhaps I may express 

 some satisfaction that in this measurement use was made 

 of the quartz fibre. 



Having now definite and accurate confirmation of the 

 existence of the fcrce produced by the action of light, or 

 rather radiation, Nichols and Hull proceed to examine the 

 question as to how far such repulsion may be competent 

 to overcome the gravitative attraction of the sun and drive 

 away the matter which pours out from the comet. It is 

 interesting to note here that Kepler put forward this very 

 idea, and that Newton, the inventor of the corpuscular 

 theory of light, looked upon the suggestion with some 

 favour. 



Coming now to this recent paper of Nichols and Hull, 

 we find first the consideration of the relation of the attrac- 

 tion by gravitation, and the repulsion by light upon par- 

 tirles of different sizes and densities. ' Densitv has no 

 influence on the action of light, while it is favourable to 

 ,u;iavitation, and therefore unfavourable to tail formation, 

 size is favourable to both, but more to gravitation than to 

 light, for if the diameter of a particle be doubled, one is 

 increased eightfold and the other only four. So size favours 

 gravitational attraction. Conversely, of course, smallness 

 favours repulsion by light, which relatively should get 

 LTreater and greater as the particles diminish in size. At 

 last, then, a degree of smallness may be reached in which 

 the repulsion by light will actually be'equal to the attraction 

 by gravitation, and such a particle would remain in space, 

 its motion unaffected by our sun. Let the diminution of 

 size continue, and then the repulsion will be in excess, and 

 if the law were to continue it would with sufficient diminu- 

 tion become relatively as large as we please. 



The law, however, does not continue. Schwarzschild has 

 shown that when the particles are small enough, light does 

 not act upon them in the same way. Owing to diffraction, 

 the effect of light is unduly great for a certain very small 

 size of particle, while it fails almost entirelv when the I 

 particle is made much smaller. Thus it is that the in- 

 definite increase in the repulsion by light as compared with ' 

 the attraction by gravitation with diminution of size of 

 particle is checked, and when, according to theory, with a I 

 particular density of particle, the light pressure' is about i 

 twenty times as great as gravitational attraction, further 

 diminution of size ceases to f;:vcur the action of light, and 

 it begins to fall off again. The distance cf the parlide 

 from the sun has no influence upon the relation between the 



two kinds cf forces, for they rise and fall together. Nichols 

 and Hull, therefore, while not denying that other causes 

 may operate, believe that light pressure is adequate to 

 account for the phenomena, and that where the material 

 coming from the head or comet proper is of two or three 

 kinds, whether of density or of size of particle, the separ- 

 ation of the two or three tails should naturally follow. 



This theory presupposes that the nucleus of a comet will 

 be able, owing to the evolution of gas under the sun's heat, 

 to send out enormous quantities of dust, the finer and 

 lighter the better, so long as it is not unduly small with 

 respect to a wave-length of light. Such dust would account 

 for any reflected solar light that the spectroscope may show, 

 but it is not easy to see how the spectrum of hydrocarbons, 

 of sodium, and of other metal, should be produced for lack 

 of temperature. It is not easy, to see why fortuitous dust 

 should be graded of such sizes as to give well separated and 

 defined tails ; it is not easy to see how the dust could be 

 produced in sufficient quantity to provide visible illumin- 

 ation to millions of millions of cubic miles of space through 

 which it may be passing at ultra-planetary velocity, even 

 though in looking through a million miles or so one grain 

 of dust in a hundred miles might suffice to supply the light. 



Other theories of the comet's tail require an electrified 

 sun, the existence of which is explained by .Arrhenius as 

 being caused by the emission by the sun of negatively 

 charged electrons which, picking up condensing gases as 

 Aitken's dust picks up moisture from the atmosphere, are 

 driven away by the light pressure. Arrhenius believes that 

 these acting on the matter in the tail would give rise to 

 the bright line spectra which have been observed. The 

 result of all this escape of negative electricity is a positively 

 charged sun, but what limits the charge in the sun it is as 

 difficult to see, as it is, why the electrostatic attraction 

 helped by gravitation does not ultimately stop the action. 

 I mav be displaying my ignorance, of which I am sufficiently 

 sensible, but I am not aware of any evidence for the 

 existence of the stream of electrified grains or drops im- 

 agined bv Arrhenius. 



Nichols and Hull, while calling to their aid the researches 

 of Schwarzschild to give them a repulsive force some twenty 

 times as great as gravitative attraction, do not seem to 

 have given due weight to the extremely small range of size 

 of particle for which this high effect is available. The 

 maximum effect for any wave-length according to Schwarz- 

 schild is produced, when the size is such that a wave-length 

 will just reach round it ; that is, with ordinary light when 

 the diameter is between one hundred thousandth and one 

 hundred and fifty thousandth of an inch. If the diameter 

 is two-and-a-half times the wave-length the action of light 

 is only equal to gravity with a material of the density of 

 water ; or again, if it is reduced to one-eighth of a wave- 

 length it again becomes equal, and in these two cases 

 there is no resultant action. With either larger or smaller 

 particles gravity rapidly gets the better of light, while the 

 high advantage of light over gravity is confined to very 

 narrow limits. 



What the sifting process can be that will give rise to 

 such a quantity of this microscopic dust we can hardly 

 expect to be told, nor why even if tTie material should in 

 some mysterious way be graded, the ungraded wave-lengths 

 of the solar spectruiri should allow of the marked separation 

 in some instances of comets' tails. 



One thing, however, they do assert, and that is that the 

 light pressure can have no action on a gas, so that if what 

 we see is considered to be gaseous the light pressure theory 

 must be thrown over for some other. 



I cannot leave this excursion of Nichols and Hull into 

 a speculative domain of science without expressing my 

 admiration of the experimental work which they have 

 accomplished, and my appreciation of the ingenuity and 

 daring with which they have attempted the hitherto 

 up.heard-of feat of making a comet. 



\\ hile the theory just referred to may be the most recent 

 it must net on that account be supposed to displace all that 

 has gone before ; the authors themselves do ret sueeest this ; 

 it is the last thing that would occur to them. They have 

 referred to the researches of Bredechin that occupy so large 

 a proportion of the annals of the Observatory of Sloscow. 



It is impossible to read even a tithe of these without feel- 

 ing that the subject of comets and their tails is one which 



NO. 1767, VOL. 68] 



