Oct. 18, 1888] 



NATURE 



59i 



displacements of india-rubber and the rotating axes, i.e. 

 the electric and the magnetic disturbances, both lie in 

 the wave-front. 



Clerk Maxwell's originally suggested representation 

 was not unlike this. 1 It consisted of a series of massive 

 wheels, connected together not by a series of elastic 

 bands but by a row of elastic particles or " idle wheels." 

 These particles represented electricity ; their displacement 

 during the period of acceleration corresponding to the 

 one-sided thickening of the elastic bands in Fitzgerald's 

 model. 



I have proposed to contemplate a double series of 

 wheels geared directly into one another, and representing 

 positive and negative electricity respectively, because it 

 seems to me that so many facts point to the existence of 

 these two entities, and because then no distinction has to 

 be drawn between one part of the medium which is ether, 

 and another part which is electricity, but the whole is 

 ether and the whole is also electricity ; while, neverthe- 

 less, a much-needed distinction can be drawn between a 

 motion of the ether as a whole, and a relative motion of 

 its component parts — a distinction between forces able to 

 move ether, i.e. to displace the centre of gravity of some 

 finite portion of it, and forces which shear it and make 

 its components slide past each other in opposite senses : 

 these latter forces being truly electromotive. 



If it be asked how the elasticity of the ether is to be 

 explained, we must turn to the vortex sponge theory, 

 suggested by Mr. Hicks 2 (principal of Firth College, Shef- 

 field), and recently elaborated by Sir William Thomson. 3 

 But this is too complicated a matter to be suited for popu- 

 lar exposition just at present. It must suffice to indicate that 

 the points here left unexplained are not necessarily at the 

 present time unexplainable, but that the explanations 

 have not yet been so completely worked out that an easy 

 grasp can be obtained of them by simple mechanical illus- 

 trations and conceptions. At the same time, the general 

 way in which motion is able to simulate the effects of 

 elasticity will be found popularly illustrated in Sir 

 William Thomson's article " Elasticity " in the " Ency- 

 clopaedia Britannica" ; and the fact that elastic rigidity 

 of a solid can be produced by impressing motion on a 

 homogeneous and otherwise structureless fluid must be 

 regarded as one of the most striking among his many 

 vital discoveries. 



We have seen that to generate radiation an electrical 

 oscillation is necessary and sufficient, and we have 

 attended mainly to one kind of electric oscillation, viz. 

 that which occurs in a condenser circuit when the distri- 

 bution of its electricity is suddenly altered— as, for 

 instance, by a discharge. But the condenser circuit need 

 not be thrown into an obviously Leyden-jar form ; one 

 may have a charged cylinder with a static charge 

 accumulated mainly at one end, and then suddenly re- 

 leased. The recoil of the charge is a true current, though 

 a weak one ; a certain amount of inertia is associated with 

 it, and accordingly oscillations will go on, the charge 

 surging from end to end of the cylinder like the water in 

 a tilted bath suddenly levelled. 



In a spherical or any other conductor, the like electric 

 oscillations may go on ; and the theory of these oscilla- 

 tions has been treated with great mathematical power 

 both by Mr. Niven and by Prof. Lamb. 4 



Essentially, however, the phenomenon is not distinct 

 from a Leyden jar or condenser circuit, for the ends of 

 the cylinder have a certain capacity, and the cylinder has 

 a certain self-induction ; the difficulty of the problem may 

 be said to consist in finding the values of these things for 

 the given case. The period of an oscillation may still be 



* Phit Mag., April i86r. 



grit. Assoc. Report, 1885, Aberdeen, p. 930. 

 3 B.A. Report, 1887, Manchester, p. 4E6. Also Phil. Mag., October 



4 Phil. Trans., 

 Proa, April 1884 



1881 and 1883. Also by Prof. J. J. Thomson, Math. Soc. 



written 27r, N /(LS); only, since L and S are both very 

 small, the "frequency" of vibration is likely to be excessive. 

 And when we come to the oscillation of an atomic charge 

 the frequency may easily surpass the rate of vibration 

 which can affect the eye. The damping out of such 

 vibrations, if left to themselves, will be also a very rapid 

 process, because the initial energy is but small. 



But whether the charge oscillates in a stationary con- 

 ductor, or whether a charged body vibrates as a whole, it 

 equally constitutes an alternating current, and can equally 

 well be treated as a source of radiation. Now, when we 

 were considering the subject of electrolysis we were led to 

 think of molecules as composed of two atoms or groups 

 of atoms, each charged with equal quantities of opposite 

 kinds of electricity. Under the influence of heat the 

 components of the molecules are set in vibration like the 

 prongs of a tuning-fork, the rate of vibration depending on 

 and being characteristic of the constants of the particular 

 molecule. The atoms being charged, however, their 

 mechanical oscillation is necessarily accompanied by an 

 electric oscillation, and so an electric radiation is excited 

 and propagated outwards. These vibrations would appear 

 to be often of the frequency suited to our retina, hence these 

 vibrating atoms indirectly constitute our usual source of 

 light. The "frequency" of the visible radiation can be 

 examined and determined by optical means (some form of 

 interference experiment, usually a diffraction grating), and 

 hence many of the rates of vibration possible to the atoms 

 of a given molecule under given circumstances become 

 known, and this is the foundation of the science of 

 spectroscopy. 



It is possible that the long duration of some kinds of 

 phosphorescence may be due to the atoms receiving 

 indirectly some of the ethereal disturbance, and so pro- 

 longing it by their inertia, instead of leaving it to the far 

 less inertia of the ether alone. It is possible also that the 

 definite emissivity of some fluorescent substances is due to 

 periods of vibration proper to their atoms, which, being 

 disturbed in an indirect way by receipt of radiation, re- 

 emit the same radiation in a modified, and, as it were, 

 laden manner. 



To get some further idea concerning the way in which 

 an oscillating charge or an oscillating charged body can 

 propagate radiation, refer back to Fig. 39, Part III. 

 (Nature, vol. xxxvii. p. 346), and imagine the rack 

 oscillating to and fro. It will produce rotatory oscillation 

 in the wheels gearing into it, these again in the next, 

 and so on. If the wheel-work were rigid, the propaga- 

 tion would go on at infinite speed to the most distant 

 wheels, but if it be elastic then the pace of propagation 

 depends on the elasticity and the density in a way we 

 have already said enough about. The line of rack 

 is the direction of electric oscillation, the axes of the 

 wheels the direction of magnetic or rotatory oscilla- 

 tion, and at right angles to both these is the direction 

 of advance of the waves. True, the diagram is not a 

 space representation, it is a mere section, and a very 

 crude suggestion of a mechanical analogy to what may be 

 taking place. 



The wheels being perfectly geared together and into the 

 rack represent an insulator or dielectric : there is no slip 

 or frictional dissipation of energy— in other words, there 

 are no true electric currents. The electric oscillation is a 

 mere displacement oscillation due to elasticity and tem- 

 porary give of the elastic wheels, whereby during each 

 era of acceleration they are thrown slightly into the state 

 represented in Fig. 46 (vol. xxxvii. p. 367) as contrasted 

 with Fig. 37 {ibid. p. 345). 



Effects of encountering a New Medium. 



Now contemplate an advancing system of waves, and 

 picture their encounter with an obstacle ; say, a medium 

 of greater density, or less elasticity, or both. If the new 

 medium is a perfect insulator, it must be considered as 



