136 LIGHT AND ELECTRICITY 



SYNTHESIS OP LIGHT. 



T shall insist less on other experiments of Hertz, more brilliant bnt 

 less instructive. Concentrating with a parabolic mirror the wave of in- 

 duction that emanates from the exciter, the (lerman physicist obtained 

 a true pencil oj" rays of electric force, susceptible of regular reflection 

 and refraction. These rays, were the ])eriod but one-milliontli of what 

 it is, would not differ from raj'S of light. We know that the sun sends 

 us several vai'ieties of ladiations, some luminiferons, since they act on 

 tlie retina, others dark, infra-red, or ultra-violet, which reveal themselves 

 in chemical andcaloritic effects. The lirst owe the qualities which ren- 

 der them sensible to us to a physiological chance. For tlie physicist, 

 the infra-red differs from red only as red differs from green; it simply 

 has a greater wave length. Tliat of the Hertzian radiations is far greater 

 still, but they are mere differences of degree, and if the ideas of Clerk 

 Maxwell are true, the illustrious professor of Bonn has effected a 

 genuine synthesis of light. 



CONCLUSION. 



Nevertheless, our admiration for such unhoi)ed-for successes must 

 not let us forget Nvhat remains to l)e accomi)lislied. Let us endeavor to 

 take exact account of tlie results definitely acijuired. 



In the first place, the velocity of direct induction through air is finite; 

 for otherwise interferences could not exist. Tlius the old electrody- 

 namics is condemned. But what is to be set up in its place? Is it to 

 be the doctrine of Maxwell, or rather some approximation to that, for 

 it w^ould be too much to sui)poso that he had ibreseeu the truth in all 

 its details'? Though the iirobabilities are accumulating, no comiilete 

 demonstration of that doctrine has ever attained. 



We can measure the wave length of the Hertzian oscillations. That 

 length is the i)roduct of the period into the velocity of propagation. 

 We should know the velocity if we knew the period; but this last is so 

 minute that we can not measure it; we can only calculate it by a for- 

 mula due to Lord Kelvin. That calculation leads to figures agreeable 

 to the theory of Maxwell; but the last doubts will onl}' be dissipated 

 when the velocity of propagation has been directly measured. (See 

 Note T.) 



But this is not all. Matters are far from being as simple as this 

 brief account of the matter would lead one to think. There are vari- 

 ous complications. 



In the lirst place, there is around the exciter a true radiation of 

 induction. The energy of the apparatus radiates abroad, and if no 

 source feeds it, it quickly dissipates itself and the oscillations are rap- 

 idly extinguished. Hence arises the phenomenon of multijde reso- 

 nance, discovered by Messrs, Sarasin and l>e la Eive. which at lirst 

 seemed irreconcilable with the theorv. 



