May 1. 1900.] 



KNOWLEDGE. 



115 



the position of eiiuilibrium. If, thoivfoiv, p is the mass 

 of vibrating; matter in unit volume, or. in other words, the 

 density of the matter, the mechuuical value of the energy 

 is^p 1-^ 



lu the ease of eireularly yx)larised li.Lfht. in wliich every 

 partiele describes a eirele with eoustaut velo"-ity, the eueriry 

 is half potential and half kiuetie. so tliat if c is the 

 constant velocity the energy is p c'-'. 



In the case of elliptically polarised light, the value 

 lies between the two. Moreover, for co-existent series 

 of waves of different periods polarised in the same plane. 

 the mechanical energy is the sum of the portions due 

 to each, from which it follows that the maximum 

 velocity is the sum of the separate velocities. 



The same reasoning applies to circularly polarised 

 light of different periods. It follows, therefore, that the 

 mechanical energy must certainly be less than the pro- 

 duct of half the mass into the square of the maximum 

 velocity acquired by a particle in the case of plane 

 polarised waves, and it may be concluded that for any 

 radiation, unless homogeneously circularly polarised, the 

 mechanical value of the disturbance is less than the 

 product of the mass into the maximum velocity of a 

 vibrating particle. 



That is to say. 4.1 x lU"^ eri,'s is less than p v ', and there- 

 fore verv much less than pY-, or p is certainly very much 



greater than ,^3 0^04)' x lO^" " 



If we assume V=100 v, which is a reasonable one to 

 make, then peonies out as somewhere about ^ x 10^-'. 

 Now the ratio of rigidity to density is equal to the square 

 of the sj>eed of transmission, which gives for the rigidity 

 i x 10"= ' X i1 X 10- ", or about f. This is small compared 

 to the density of any known solid. Steel is the most rigid 

 substance kaown to us, and its rigidity is as bigh as 

 8x10". 



It is not only in free space that luminous and other 

 electrical vibrations are transmitted by ether. Water 

 and other fluids, for example, transmit light, but it 

 cannot be the fluid which acts as the medium, for it has 

 no rigidity, and is therefore incapable of transmitting 

 transverse vibrations. Even in transparent solids the 

 waves must be transmitted by ether penetrating the 

 interstices of the matter composing them, for the rate 

 of transmission is far too great for the matter itself 

 to be the medium. 



The ether, however, within different kinds of matter 

 is largely modified to an extent depending on the sub- 

 stance. For example, in heavy glass the speed of trans- 

 mission of a luminous wave is only about two-thirds 

 of the speed in free space. The ether must, therefore, 

 either have its density increased or its rigidity 

 diminished by the presence of the particles of glass. 



Many considerations appear to me to favour the latter 

 hypothesis in preference to the former one. If the ether 

 is capable of having its density varied it must be com- 

 pressible, and therefore its structure must be molecular, 

 and these molecules must be elastic, and then if we are 

 to adhere to our plan of assuming that evei-y action 

 between distant bodies is due to actual pushing or 

 pulling of bodies actually in contact with them, we 

 shall require a second ether to explain the elasticity of 

 the molecules of the first. 



We therefore seem to be driven to the conclusion 

 that the ether is to be regarded as continuous and there- 

 fore incompressible, so that the modification of ether in 

 contact with matter must consist in a diminution of 

 rigidity and not in an increase of density. 



This conclusion appears to me to be strongly confirmed 

 by the simple explanation which it gives of opacity. 

 There is no such thing as a perfectly opaque body, but 

 some come very neai- it, and on our theory, which is 

 practically that of McCuUagh, the explanation is that 

 in such bodies the rigidity of the ether ap])ioaclics the 

 vanishing point. If we adopted Fresnel's theory of the 

 increase of density of ether in contact with matter we 

 should have to supjjose the density of the ether in prac- 

 ticallv ojj.ique bodies to be increased to an enormous 

 extent. 



Another ])oiiit in favour of this view is that if 

 we assume with McCullagli that the diminution in 

 rigidity is due to a sort of straining of the ether towards 

 the particles of matter we get at something like the ex- 

 planation of gravitation, for under those circumstances 

 two bodies would tend to draw together. Objectors to 

 the ether on the ground of the complication involved 

 in the co-existence of two apparently so distinct things 

 as matter and ether may be interested to learn that 

 Lord Kelvin has suggested a simplification of a very 

 beautiful character. 



While fluids at rest have no rigidity, portions of them 

 may become rigid by being set in rapid motion, as is 

 well illustrated by the smoke rings which some smokers 

 are very skilful in blowing from their mouths and which 

 may easily be produced in air, water, and other fluids. 



Lord Kelvin made the beautiful suggestion that the 

 apparently unchangeable atoms of different kinds might 

 simply be vortex rings of various shapes in the ether, 

 which from this point of view must act as a perfect 

 fluid. _ _ 



En-ata to last Article, p. 25, column 2, line 2(i, read " this medium 

 is relativelt/ to its densiti/ far more rigid than steel " ; p. 26, column 2, 

 line 13, for " electric bodies " read " electric waves." 



DROPS AND THEIR SPLASHES. 



The beauty and scientific meaning of familiar pheno- 

 mena cannot be gauged by popular views regarding 

 them. It would be difficult to imagine anything more 

 commonplace to the ordinary observer than the splash 

 of a drop, yet Professor A. M. Worthington s researches, 

 extending over many years as they have done, and the 

 first series of which were but recently completed, show 

 that this apparently simple occurrence, when examined 

 by the refined methods which science makes possible, 

 is really a succession of bewilderingly beautiful phases, 

 which for their complete intei-jjretation require the re- 

 sources of higher mathematical analysis. The same dis- 

 tinguished investigator has demonstrated that a variety 

 of allied phenomena, while fundamentally dependent 

 upon the same properties of matter, are all characterized 

 by their individual peculiarities of changing forms, 

 which can be reproduced at will by the experimenter. 



It is not necessai-y in this place to trace the develop- 

 ment of the perfected form of apparatus used in the 

 most recent experiments, for the steps by which the final 

 disposition of the instruments was reached may be fol- 

 lowed in the " Proceedings ' and " Philosophical Trans- 

 actions of the Royal Society." The essence of the 

 problem which Prof. Worthington had to solve can be 

 very briefly stated. How could a drop of definite size 

 be allowed to fall fz-om a known height in comparative 

 darkness upon a surface, and be illuminated by a flash 

 of exceedingly short duration at any desired instant, 

 at any particular stage of the impact of the drop, so as 

 to exclude all other stages previous and subsequent to 



