JAMES CLERK MAXWELL 329 



the velocity of propagation of induction across air is finite, but also 

 that it is equal to the velocity of the wave propagated along a wire, 

 conformably to the ideas of Maxwell. 



SYNTHESIS OF LIGHT 



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

 less instructive. Concentrating with a parabolic mirror the wave of 

 induction that emanates from the exciter, the German physicist ob- 

 tained a true pencil of rays of electric force, susceptible of regular re- 

 flection and refraction. These rays, were the period but one-millionth 

 of what it is, would not differ from rays of light. We know that the 

 sun sends us several varieties of radiations, some luminiferous, since 

 they act on the retina, others dark, infra-red, or ultra-violet, which 

 reveal themselves in chemical and calorific effects. The first owe the 

 qualities which render them sensible to us to a physiological chance. 

 For the physicist, the infra-red differs from red only as red differs 

 from green ; it simply has a greater wave length. That of the Hertz- 

 ian radiations is far greater still, but they are mere differences of de- 

 gree, and if the ideas of Clerk Maxwell are true, the illustrious pro- 

 fessor of Bonn has effected a genuine synthesis of light. 



CONCLUSION 



Nevertheless, our admiration for such unhoped-for successes must 

 not let us forget what remains to be accomplished. Let us endeavor 

 to take exact account of the results definitely acquired. 



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

 finite; for otherwise interferences could not exist. Thus the old 

 electro-dynamics 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 would be too much to suppose that he had foreseen the 

 truth in all its details ? Though the probabilities are accumulating, no 

 complete demonstration of that doctrine has ever attained. 



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

 length is the product 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 cannot measure it ; we can only calculate it by a 



