May 3. 1894] 



NATURE 



II 



studied by Hertz, are fulfilled, this unclamping is sudden 

 enough to enable oscillations to be produced. 



The apparatus, called an " exciter," produces currents 

 which change their direction from 100,000,000 to 

 1,000,000,000 times per second. Because of this extreme 

 frequency they can produce inductive eflects at a great 

 distance. In order to render these effects simple, another 

 electric pendulum, called a "resonator," is employed. 

 In this new pendulum, the cut and the coil, which only 

 serve for the unclamping, are suppressed; the two con- 

 ductors reduce themselves to two very small spheres, and 

 the wire is bent back in a circle in a way to approach 

 the spheres to each other. 



The induction due to the exciter will put this resonator 

 in vibration the more easily as the periods of the two are 

 less different. At certain phases of the vibration, the 

 difference of potential of the two spheres will be large 

 enough to produce sparks. 



Production 0/ Interferences. 

 We have thus an instrument which shows the 

 effects of an inductive wave emitted from the exciter. 

 We can study what happens in two ways : either 

 expose the resonator to the direct induction of the ex- 

 citer at a great distance, or else make this induction work 

 at a short distance on a long conducting wire, along 

 which the electric wave will go, and which will work in 

 its turn by induction at a short distance on the 

 resonator. 



Whether the wave propagates itself along a wire or 

 across the air, one can produce interferences by reflection. 

 In the first case, it will reflect itself at the extremity of the 

 ' wire, which it will follow again in an inverse direction ; 

 I in the second, it will reflect itself on a metallic leaf 

 j which acts as a mirror. In the two cases the reflected 

 t wave will interfere with the direct wave, and we can 

 j find places where the spark of the resonator will cease 

 '■ to pass. 



The experiments made with the long wire are easier ; 

 they furnish us with very precious instruction, but they 

 will not serve as e.x'perivienta cruets; for in the 

 I old as well as the modern theory, the quickness of 

 I an electric wave along a wire must be equal to that of 

 ! light. The experiments on the direct induction at a 

 ■great distance are, on the contrary, decisive. They show 

 that not only the quickness of propagation of induction 

 across the air is finite, but that it is equal to the quick- 

 ness of the wave propagated along a wire, complying with 

 the ideas of Maxwell. 



Synthesis of Light. 

 I I shall insist less on other experiments of Hertz, more 

 brilliant, but less instructive. Concentrating with a para- 

 Ibolic mirror the wave of induction taken from the exciter, 

 |the German savant obtains a veritable cluster of 

 electric rays, capable of reflecting and refracting them- 

 selves regularly. The rays, if the period, already so small, 

 were a million times shorter still would not differ from the 

 luminous rays. We know that the sun gives out 

 several kinds of radiation, some luminous because they 

 act on the retina, others obscure ultra-violet or infra- 

 red, which manifest themselves by their chemical or 

 calorific effects. The first only owe their qualities, which 

 rnake them appear to us of a different nature, to a kind of 

 physiological chance. To the physicist the infra-red 

 ioes not differ more from the red, than the red from the 

 ween ; the length of a wave is only greater ; those of the 

 pertzian radiations are much greater still, but there are 

 pnly differences of degree, and one may say, if Max- 

 ell's theories are true, that the illustrious Professor of 

 3onn has realised a veritable synthesis of light. 



Conclusions. 

 But our admiration for so much unhoped-for success 

 hust not make us forget the progress which still remains 



NO. 1279, VOL. 50] 



to be accomplished. Let us therefore try to exactly 

 summarise the results which are definitely attained. 



First, the velocity of direct induction across the air 

 is finite, without which the interferences would be im- 

 possible. The old electro-dynamics are therefore con- 

 demned. What must one put in its place.- Is it 

 Maxwell's theory (or at least something approaching it, 

 for one would not expect the divination of the Eng- 

 lish savant to have foreseen the truth in all its details) ? 

 Although the probabilities accumulate, the complete de- 

 monstration is not yet reached. 



We can measure the length of a wave of hertzian 

 oscillations ; this length is the product of the period 

 by the velocity of propagation. We should, therefore, 

 know this velocity if we knew the period ; but this last 

 is so small that we cannot measure it : we can only cal- 

 culate it by a formula due to Lord Kelvin. This calcula- 

 tion leads to numbers which agree with Maxwell's theory; 

 but the last doubts will only be done away with when the 

 velocity of propagation has been directly measured. 



This is not all : things are far from being so simple as 

 one might think, from the above short account. Diverse 

 circumstances come to complicate them. 



First, there is round the exciter a radiation of 

 induction ; the energy of this apparatus radiates, 

 therefore, externally, and as no fresh source comes to 

 supply it, it soon disperses, and the oscillations die out 

 very rapidly. It is here that one must look for the 

 explanation of the phenomenon of multiple resonance, 

 which was discovered by MM. Sarasin and De !a Rive, 

 and which at first appeared irreconcilable with the 

 theory. 



On the other hand, we know that light does not pre- 

 cisely follow the laws of geometrical optics, and the 

 difference which produces diffraction, is more consider- 

 able as the length of the wave is greater. With the great 

 length of the hertzian undulations these phenomena must 

 assume an enormous importance, and trouble everything. 

 No doubt it is fortunate, for the moment at least, that 

 our means of observation are so coarse, otherwise the 

 simplicity which seduced us at the first sight would give 

 place to a labyrinth where we should be lost. It is from 

 this probably that different anomalies arise, which have 

 hitherto not been explained. It is also for this reason 

 that the experiments on the refraction of rays of electric 

 force have, as I said above, but little demonstrative 

 worth. 



There still remains a difficulty which is more serious, 

 but which is no doubt not insurmountable. .According 

 to Maxwell, the coefficient of electrostatic induction of a 

 transparent body ought to be equal to the square of its 

 index of refraction. This is not so ; the bodies which 

 follow Maxwell's law are exceptions. We are evidently 

 in the presence of phenomena much more complex than 

 w^e thought at first ; but one has not been able to explain 

 anything, and the experiments themselves are con- 

 tradictory. 



There still remains, therefore, much to be done ; the 

 identity of light and electricity is from to-day something 

 more than a seducing hypothesis : it is a probable truth, 

 but it is not as yet a proved troth. 



THE RECEXT WORK OF THE CATARACT 

 CONSTRUCT/OA' COMPANY. 



SOME arrangements recently made by the Cataract 

 Construction Company show that the works are ex- 

 tending in a very satisfactory manner. The Niagara 

 Falls Paper Company is now well under way. They make 

 paper from wood pulp, and a large amount of power is 

 used for grinding the trees down into pulp. They have 

 fixed turbines in their own wheel-pit, and take water from 



