Azigust 30, 1888] 



NATURE 



4i7 



some form of alternating machine, their frequency is, of 

 course, arbitrary ; but if they be automatically caused by 

 the recoil of a given Leyden jar in a given circuit, their 

 frequency is, as we have already said, 



1 



per second ; 



where L is the electrical inertia or self-induction of the 

 circuit, and where S is the capacity or reciprocal of the 

 elasticity constant of the jar. 



It is not convenient here to go into the determination 

 of the quantity L, but roughly one may say that for an 

 ordinary open single-loop circuit it is a quantity somewhat 

 comparable with twelve or fifteen times its circumference 

 multiplied by the constant fi. 



The value of S has to do with the area and thickness 



A 



of the condenser, being, as is well known, ■ multiplied 



by the constant K. 



The product LS contains therefore two factors, each 

 of linear dimensions, expressing the sizes of circuit 

 and jar, and likewise contains a factor ^K expressing 

 the properties of the surrounding medium. Hence, so 

 far as the ether is concerned, the above expression 

 for frequency of vibration demands only a knowledge of 

 the. product of its two constants K and p, and since this 

 is known by the previous velocity experiment, it is easy to 

 calculate the rate of oscillation of any given condenser 

 discharge. It is also easy to calculate the wave-length ; 

 for if there are n waves produced per second, and 

 each travels with the velocity v, the length of each 



• 71 

 wave is -. 

 n 



Hence the wave-length is 2ir „ /( - . — Y 



Now, if we go through these numerical calculations for 

 an ordinary Leyden jar and discharger, we shall find 

 waves something like, say, 50 or 100 yards long. They 

 may plainly be of any length, according to the size of the 

 jar and the size of the circuit. The bigger both these are 

 the longer will be the waves. 



A condenser of 1 microfarad capacity, discharging 

 through a coil of self-induction 1 secohm, will give rise 

 to ether waves 1900 kilometres or 1200 miles long. 



A common pint Leyden jar discharging through a pair 

 of tongs may start a system of ether waves each not 

 longer than about 15 or 20 metres. 



A tiny thimble-sized jar overflowing its edge may 

 propagate waves only about 2 or 3 feet long. 



The oscillations of current thus recognized as setting up 

 waves have only a small duration, unless there is some 

 means of maintaining them. How long they will last 

 depends upon the conductivity of the circuit ; but even in 

 a circuit of infinite conductivity they must die out if left 

 to themselves, from the mere fact that they dissipate their 

 energy by radiation. One may get 100 or 1000, or perhaps 

 even 100,000, perceptible oscillations of gradually de- 

 creasing amplitude, but the rate of oscillation is so great 

 that their whole duration may still be an extremely small 

 fraction of a second. For instance, to produce ether 

 waves a metre in length requires 300,000,000 oscillations 

 per second. 



To keep up continuous radiation naturally requires a 

 supply of energy, and unless it is so supplied the radiation 

 rapidly ceases. Commercial alternating machines are 

 artificial and cumbrous contrivances for maintaining elec- 

 trical vibrations in circuits of finite resistance, and in 

 despite of loss by radiation. 



In most commercial circuits the loss by radiation is 

 probably so small a fraction of the whole dissipation of 

 energy as to be practically negligible ; but one is, of course, 

 not limited to the consideration of commercial circuits or 

 to alternating machines as at present invented and used. 



It may be possible to devise some less direct method — 

 some chemical method, perhaps — for supplying energy to 

 an oscillating circuit, and so converting what would be a 

 mere discharge or flash into a continuous source of 

 radiation. 



So far we have only considered ordinary practicable 

 electrical circuits, and have found their waves in all cases 

 pretty long, but getting distinctly shorter the smaller we 

 take the circuit. Continue the process of reduction in 

 size further, and ask what sized circuit will give waves 

 6000 tenth-metres (three-fifths of a micron, or 25 millionths 



of an inch) long. We have only to put 2tc /( — . _ j = 



o"oooo6, and we find that the necessary circuit must have 

 a self-induction in electro-magnetic units, and a capacity 

 in electrostatic units, such that their geometric mean is 

 io -3 centimetre (one-tenth of a micron). This gives us at 

 once something of atomic dimensions for the circuit, and 

 suggests immediately that those short ethereal waves which 

 are able to affect the retina, and which we are accustomed 

 to call " light," may be really excited by electrical 

 oscillations or surgings in circuits of atomic dimensions. 



If after the vibrations are once excited there is no source 

 of energy competent to maintain them, the light production 

 will soon cease, and we shall have merely the temporary 

 phenomenon of phosphorescence ; but if there is an avail- 

 able supply of suitable energy, the electrical vibrations 

 may continue, and the radiation may become no longer an 

 evanescent brightness, but a steady and permanent glow. 



Velocity of Electrical Radiation compared with Velocity 

 of Light. 



We have thus imagined the now well-known Maxwellian 

 theory of light, viz. that it is produced by electrical 

 vibrations, and that its waves are electrical waves. 



But what justification is there for such an hypothesis 

 beyond the mere fact which we have here insisted on, viz. 

 that waves in all respects like light-waves except size, i.e. 

 transverse vibrations travelling at a certain pace through 

 ether, can certainly be produced temporarily in practicable 

 circuits by familiar and very simple means, and could be 

 produced of exactly the length proper to any given kind of 

 light if only it were feasible to deal with circuits ultra- 

 microscopic in size ? The simplest point to consider is : 

 Does light travel at the same speed as the electrical dis- 

 turbances we have been considering? We described one 

 method of measuring how fast electrical radiation travels 

 in free space, and there are many other methods : the 

 result was 300,000 kilometres per second. 



Methods of measuring the velocity of light have long 

 been known, and the result of those measurements in 

 free space or air is likewise 300,000 kilometres a second. 

 The two velocities agree in free space. Hence surely 

 light and electrical radiation are identical. 



But there is a further test. The speed of electrical 

 radiation was not the same in all media : it depended on 

 the electrical elasticity and the ethereal density of the 

 transparent substance ; in other words, it was equal to 

 the reciprocal of the geometric mean of its specific 

 inductive capacity and its magnetic permeability — 



_ I 

 v ~ 7(K# 



Now, although the absolute value of neither K nor /x 

 is known, yet their values relatively to air are often 

 measured and are known for most substances. 



Also, it is easy to compare the pace at which light goes 

 through any substance with its velocity in free space : the 

 operation is called finding the refractive index of a sub- 

 stance. The refractive index means, in fact, simply the 

 ratio of the velocity of light in space to its velocity in the 

 given substance. The reciprocal of the index of refrac- 

 tion is therefore the relative velocity of light. Calling 

 the index of refraction n, therefore, we ought, if the 



