TRANSACTIONS OF SECTION A. 529 



Having now definite and accurate confirmation of the existence of tlie force 

 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 attraction by gravitation, and the repulsion by 

 light upon particles of dift'ereut sizes and densities. Density has no influence on 

 the action of light, while it is favourable to gravitation, and therefore unfavour- 

 able 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 greater 

 and greater as the particles diminish in size. At last, then, a degree of smallnees 

 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 

 unatiected by our sun. Let the diminution of size continue, and then the 

 repulsion will be in excess, and if the law were <o continue it would with 

 sufficient diminution 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 upontbem in the same way. 

 Owing to diffraction, the efl'ect of light is unduly great for a certain very small 

 eize of particle, while it fails almost entirely when the particle is made much 

 smaller. Thus it is that the indefinite 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 particular density of particle, the 

 light pressure is about twenty times as great as gravitational attraction, further 

 dimiuution of size ceases to favour the action of light, and it begins to fall off 

 again. The distance of the particle from the sun has no influence upon the 

 relation between the two kinds of forces, lor 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 separation 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 he 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 illumination to 

 milUons 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 Aitkins'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 difficult to see, as it is, why the electrostatic attraction 

 helped by gravitation does not ultimately .«top the action. 



Nichols and Hull, while calling to their aid the researches of Schwarz-. 

 1903. M M 



