530 REPORT— 1903. 



schild to give them a repulsive force some twenty times as great as gravita- 

 tive attraction, do not seem to have given due weight to the extremely small 

 range of size of particle for which this high eifect is available. The maximum 

 effect for any wave-length according to Schwarzschild 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 the material 

 should in some mysterious way be graded, the ungraded wave-lengths of the solar 

 spectrum should allow of the marked separation iu 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, or of my appreciation of the ingenuity and daring with which 

 they have attempted the hitherto unheard-of feat of making a comet. 



While the theory just referred to may be the most recent it must not on that 

 account be supposed to displace all that has gone before ; the authors themselves 

 do not suggest 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 Moscow. 



It is impossible to read even a tithe of these without feeling that the subject 

 of comets and their tails is one which Bredechin, by his amazing industry,'has 

 made his own property, and that any stranger casually passing by and taking a 

 random shot should receive the severe penalty awarded to poachers in this 

 country. Bredechin has dealt unmercifully — I do not say unjustly — with the 

 author of at least one such random theory. 



It is therefore with the greater diffidence and more urgent plea for forbear- 

 ance that I venture to draw certain parallels and hazard certain suggestions 

 which I admit freely have not reached a stage at which detailed compariscns with 

 known comets are possible. 



It does not seem possible now to contemplate the phenomena of the comet, of the 

 divided tails, of their tenuity and transparency, of the pale luminosity, partly 

 reflected solar light, partly light as from a glowing gas ; of the gradual wearing 

 out and disappearance of those comets which constantly pay visits to solar regions, 

 with all the mysteries of radium now so much in evidence without tracing the 

 features in which they resemble one another. By radium, of course, I mean any 

 material with the remarkable radio-active properties that radium exhibits with 

 such pre-eminent splendour, whether known in the laboratory or cot. 



How many physicists have been peering at comets through radium spectacles, 

 or how many astronomers detect the sparkle of radium in the fairy tresses of 

 their hirsute stars I know not. One writer, however, T. C. Chamberlin, so long 

 ago as July 1901, looked upon a connexion between radio-active materials such as 

 were then known and comets as at least worth considering. Chamberhn's paper 

 in the ' Astrophysical Journal' was mainly on the tidal disruption of gravitating 

 bodies and the possible evolution of comets, nebulae and meteorites, and he did not 

 pursue this consideration in any detail ; indeed, the enormous accumulation of new 

 properties of radium was not then available. 



Whatever may be imagined as to the constitution of a comet, difficulties still 

 remain. All I suggest now is that the curious properties of radium and of 

 similar bodies should be kept in mind. Radium at least supplies the means by 



