114 



KNOWLEDGE. 



[May 1, 1900. 



elasticity, or resistance to change of volume, they have 

 no rigidity, or resistance to change of shape, and sub- 

 stances without rigidity can only transmit longitudinal 

 vibrations, the transverse vibrations being entirely due 

 to resistance to shearing, that is, to the sliding of ons 

 portion of the substance over another. 



It has long been known from the phenomena of light 

 that the vibrations are entirely transverse, that is to 

 say, any pai-ticle of the vibrating medium i-emains 

 throughout its motion always in the plane pei-pendicular 

 to the direction of transmission of the ray of light, the 

 longitudinal vibrations being non-existent. No explana- 

 tion of this suppression of the longitudinal vibrations 

 was obtained until Maxwell showed theoretically that 

 this was characteristic of electro-magnetic waves, and 

 suggested the probability of light waves being simply 

 electro-magnetic waves having wave lengths betw.ien 

 the limits within which the human eye was capable of 

 responding to them. 



Between fifty and sixty years ago that great philo- 

 sopher and experimentalist, Michael Faraday, seems to 

 have had some kind or instinctive glimmering of an idea 

 that there was some connection between electricity and 

 light. In the then state of knowledge there was nothing 

 apparently to warrant it, but he tried a number of 

 experiments upon the effects of electric and magnetic 

 fields upon rays of light before he obtained any result. 



He allowed a beam of plane polarised liglit to pass 

 through holes in the poles of a powerful electro-magnet, 

 so that the direction of transmission of the ray was 

 parallel to the lines of force of the magnetic field. A 

 very dense kind of glass containing borate of lead, a 

 glass which Faraday had himself discovered and made 

 some yeais before, was then placed between the poles, 

 when it was found that if an analyser was so aiTanged 

 as to stop all the light before the magnet was excited, 

 then on excitation taking place there was a slight 

 brightening of the field which could be reduced to 

 darkness again by slightly rotating the analyser. 



Neither Faraday nor anyone else was able at the time 

 to account for a fact obtained through the coincidence 

 of a number of circumstances all requisite for success, 

 though not one of them could have been predicted, and 

 which furnishes a wonderful example of the thorough- 

 ness and utter disregard of repeated failures which was 

 one of the leading characteristics of Faraday's experi- 

 mental work. 



The meaning of this experiment was fii-st pointed out 

 by Sir William Thomson, now Lord Kelvin, and its 

 important consequences were fully investigated by 

 Maxwell, who in all probability was led by it to formu- 

 late his electro-magnetic theory of light. 



I have already pointed out that the vibrations form- 

 ing a ray of light are all in a plane perpendicular to the 

 direction of the ray. In general the vibrations take 

 place in all possible directions in this plane, but it is 

 possible by allowing a beam of light to pass through 

 certain crystals, and by other means, to break up these 

 vibrations in all possible directions in the plane into 

 vibrations in two directions at right angles to each 

 other, and it is further possible by simple means to get 

 rid of one set of vibrations, leaving only vibrations 

 which are all perpendicular to a plane containing the 

 ray and which is known as the plane of polarisation, 

 the light being said to be plane polarised. 



Faraday's discoveiy was that it was possible by means 

 of a magnetic field to produce rotation of the plane of 

 polarisation. 



Maxwell called attention to the fact that the observed 

 velocity of light was, within the limits of errors of 

 obsei-vation, identical with the rate of propagation of 

 an electro-magnetic disturbance deduced theoretically 

 from certain electrical measurements, and cited other 

 experimental facts in its favour ; and many other facts 

 since discovered have confirmed Maxwell s conclusions, 

 more particularly the work of Hertz, which I shall 

 consider later in some detail, and in which he demon- 

 strated experimentally that electro-magnetically excited 

 waves could be made to interfere with each other and 

 could be reflected and refracted exactly like light 

 waves. 



Heat and light are therefore found to be mere special 

 cases of electro-magnetic waves which may vary through 

 all gradations of wave lengths varying from thousands 

 of miles down at any rate to a few hundred thousandths 

 of an inch in the case of light waves, and the great 

 electro-magnetic spectrum extends, we know, far beyond 

 this, for we can detect by their effect on photographic 

 chemicals the existence of waves far beyond the violet 

 end of the spectrum, that is to say of waves shorter than 

 the shortest light waves which the eye can perceive. 



Lord Kelvin, in a paper published in the " Trans- 

 actions of the Royal Society of Edinburgh " in May, 

 1854, has shown how a probable minimum limit may be 

 assigned to the density of the ether. 



The French physicist, Pouillet, as the result of a 

 series of carefully-made measurements, had found that 

 the heating effect of direct sunlight fallng on a surface 

 of a square centimetre at the distance of the earth from 

 the sun amounted to L7633 gramme Centigrade units 

 of heat per minute, or 1.234x10^ ergs per second. This 

 would evidently be the amount of energy due to sun- 

 light contained in a prism with a base having an area 

 of a square centimetre and with a height equal to the 

 velocity of light in centimetres per second, viz., 3.004 x 10'", 

 which gives as the energy per cubic centimetre per 

 second : 



1.234 xl0» , , ,,.^, 



= 4.1 X 10 ^ ergs. 



3.004 X 10 1" " 



Lord Kelvin deduces from this datum a superior limit 

 to the velocity of a vibrating particle of the medium 

 transmitting radiant heat or light, on the assumption 

 that the amplitude of vibration is a small fi-action of the 

 wave length and that the maximum velocity of a vibrating 

 pai'ticle is small compared with the speed of propaga- 

 tion of waves. The first assumption is certainly justi- 

 fiable, and the second follows from it, for considering 

 the case of plane polarised light where the vibration is 

 a simple harmonic one, if V be the velocity of wave 

 transmission, v the maximum velocity of a vibratini^ 

 particle, A the semi-amplitude or distance of the vi- 

 brating particle at the extremity of an excursion from 

 the position of equilibrium, and X the wave length, 

 then it is known that 



V J, A 



— = 2 Jr 



V \ 



Now the whole mechanical energy of homogeneous 

 plane polarised light in an infinitely small space 

 containing only particles sensibly in the same plane of 

 vibration is entirely potential when the particles are at 

 rest at either end of an excursion, entirely kinetic when 

 the particles ai-e in the position of equilibrium, and 

 partly potential and pai-tly kinetic in all other cases. 



This energy being constant in amount is equal to ^ m v-, 

 where m is the mass in vibration, for v is a maximum in 



