November 17, 1898] 



NA TURE 



57 



great in comparison with the radius of the shell), the 

 wave-length will be approximately 3 kilometres.' In- 

 crease the frequency now to 1000 periods per second : at 

 distances of hundreds of metres the wave-length will be 

 about 3 metres. Increase now the frequency to 10'' 

 periods per second ; the wave-length will be 3 millim., 

 and this not only at distances of several times the radius 

 of the shell, but throughout the elastic medium from close 

 to the outer surface of the shell ; because the wave-length 

 now is a small fraction of the radius of the shell. In- 

 crease the frequency further to 1000 xio'^ periods per 

 second ; the wave-length will be 3 x lo"^ of a millim., or 

 3 mikroms,- if, as in all probability is the case, the distance 

 between the centres of contiguous molecules in glass and 

 in iron is less than a five-hundredth of a mikrom. But 

 it is probable that the distance between centres of con- 

 tiguous molecules in glass and in iron is greater than 

 10"^ of a mikrom, and therefore it is probable that 

 neither of these solids can transmit waves of distortional 

 motion of their own ponderable matter, of so short a 

 wave-length as 10""' of a mikrom. Hence it is probable 

 that if we increase the frequency of the rotational vibra- 

 tions of our shell to one hundred thousand times 1000 

 X 10'', that is to say, to 100 x 10'-, no distortional wave of 

 motion of the ponderable matter can be transmitted out- 

 wards ; but it seems quite certain that distortional waves 

 of radiant heat in aether will be produced close to the 

 boundary of the vibrating shell, although it is also 

 probable that if the surrounding solid is either glass or 

 iron, these waves will not be transmitted far outwards, 

 but will be absorbed, that is to say, converted into non- 

 undulatory thermal motions, within a few mikroms of 

 their origin. 



Lastly, suppose the elastic solid around our oscillating 

 shell to be a concentric spherical shell of homogeneous 

 glass of a few centimetres, or a few metres, thickness and 

 of refractive inde.x 1-5 for D light. Let the frequency of 

 the oscillations be increased to 5-092 x 10'^ periods per 

 second, or its period reduced to -589212 of a michron : 

 homogeneous yellow light of period equal to the mean of 

 the periods of the two sodium lines will be propagated 

 outwards through the glass with wave-length of aboiit 

 ]t x -589212 of a mikrom, and out from the glass into air 

 with wave-length of -589212 of a mikrom. The light will 

 be of maximum intensity in the equatorial plane and 

 zero in either direction along the axis, and its plane of 

 polarisation will be everywhere the meridional plane. It 

 is interesting to remark that the axis of rotation of the 

 aether for this case coincides everywhere with the line of 

 vibration of the aether in the case first considered ; that 

 is to say, in the case in which the shell vibrated to and fro 

 in a straight line, instead of, as in the second case, rotating 

 through an infinitesimal angle round the same line. 



A full mathematical investigation of the motion of the 

 elastic medium at all distances from the originating shell, 



1 "Math, and Phys. Papers," vol. iii. art. civ. p. 522. 



'- For a small unit of length Langley, fourteen years ago, used with great 

 advantage and convenience the word " mikron *' to denote the millionth of a 

 metre. The letter » has no place in the metrical system, and I venture to 

 suggest a change of spelling to " mikrom " for the millionth of a inetre, after 

 the analogy of the English usage for millionths (mikrohm, mikro-ampere, 

 mikrovolt). For a conveniently small corresponding unit of time 1 further 

 venture to suggest " michron " to denote the period of vibration of light 

 whose wave-length in aether is i mikrom. Thus, the velocity of light in 

 aether being 3 x lo* metres per second, the michron is i X lo-'-" of a second, 

 and the velocity of light is i mikrom of space per michron of time. Thus 

 the frequency of the highest ultra-violet light investigated by Schumann 

 ('I of a mikrom wave-length, S:fz;ingsl'fr. d, k, Gesellsch. d. IVisscnsch, zu 

 Wien,6\. pp. 415 and 625, 1893) is 10 periods per michron of time. The 

 period of sodium light (mean of lines D) is -589212 of a michron ; the periods 

 of the " Reststrahlen " of rocksalt and sylvin found' by Rubens and 

 Aschkinass (Wicd. Ann.,W\. (1898, p. 241) are 51-2 and 6i-i michrons 

 respectively. 



No practical inconvenience can ever arise from any possible confusion, or 

 momentary forgetfulness, in respect to the similarity of sound between 

 michrons of time and mikroms of space. — K. 



NO. I 5 16, VOL. 59] 



for each of the cases of I. fi) and I. (2), will be found in 

 a volume containing my Baltimore Lectures on "Mole- 

 cular Dynamics and the Wave-Theory of Light," soon, I 

 hope, to be published. 



II. An electrical analogy for I. (i) is presented by sub- 

 stituting for our massless shell an ideally rigid, infinitely 

 massive shell of glass or other non-conductor of electricity, 

 and for our massive platinum nucleus a massless non- 

 conducting globe electrified with a given quantity of 

 electricity. For simplicity we shall suppose our apparatus 

 to be surrounded by air or aether. Vibrations to and fro 

 in a straight line are to be maintained by force between, 

 shell and nucleus as in I. (i). Or, consider simply a 

 fixed solid non conducting globe coated with two circular 

 caps of metal, leaving an equatorial non-conducting zone 

 between them, and let thin wires from a distant alternate- 

 current dynamo, or electrostatic inductor, give period- 

 ically varying opposite electrifications to the two caps. 

 For moderate frequencies we have a periodic variation 

 of electrostatic force in the air or aether surrounding the 

 apparatus, which we can readily follow in imagination, 

 and can measure by proper electrostatic measuring ap- 

 paratus. Its phase, with moderate frequencies, is very 

 exactly the same as that of the electric vibrator. Now 

 suppose the frequency of the vibrator to be raised to 

 several hundred million million periods per second. We 

 shall have polarised light proceeding as if from an ideal 

 point-source at the centre of the vibrator and answering 

 fully to the description of 1. (i). Does the phase of 

 variation of the electrostatic force in the axial line out- 

 side the apparatus remain exactly the same as that of 

 the vibrator? An affirmative answer to this question 

 would mean that the velocity of propagation of electro- 

 static force is infinite. A negative answer would mean 

 that there is a finite velocity of propagation for electro- 

 static force. This velocity, according to views regarding 

 conceivable qualities of aether described in my article 

 " On the Reflection and Refraction of Light " {Phil. Mag., 

 vol. xxvi. 1888) might be greater than, equal to, or less, 

 than the velocity of light. 



III. The shell and interior electrified non-conducting 

 massless globe being the same as in II., let now a forcive 

 be applied between shell and nucleus to produce rota- 

 tional oscillations as in 1. (2). When the frequency of 

 the oscillations is moderate, there will be no alteratiork 

 of the electrostatic force and no perceptible magnetic 

 force in the air or aether around our apparatus. Let now 

 the frequency be raised to several hundred million million 

 periods per second ; we shall have visible polarised light 

 proceeding as if from an ideal point-source at the centre 

 and answering fully to the description of the light of I. (2) 

 The same result would be obtained by taking simply a 

 fixed solid non-conducting globe and laying on wire orn 

 its surface approximately along the circumferences of 

 equidistant circles of latitude, and, by the use of a distant 

 source (as in II.) sending an alternate current through 

 this wire. In this case, while there is no manifestation 

 of electrostatic force, there is strong alternating magnetic 

 force, which in the space outside the globe is as it from 

 an ideal infinitesimal magnet with alternating magnetisa- 

 tion, placed at the centre of the globe and with its 

 magnetic axis in our axial line. 



THE SERUM TREATMENT OF RINDERPEST. 



THE Report of the Colonial Veterinary Surgeon and 

 the Assistant \'eterinary Surgeons of the Cape of 

 Good Hope is at the present time a document of very 

 considerable interest, as it is possible from the facts 

 there set forth to form some definite opinion as to the 

 efficacy of the various methods of protection and treat- 

 ment against rinderpest that have been recommended in 

 the Colony. It appears that there is now a possibility of 

 completely eradicating rinderpest from South Africa, not, 



