RADIATION. 33 



altering the periods of the old vibrations, or the times 

 in which they were executed. But besides intensifying 

 the old vibrations the current generates new and more 

 rapid ones, and when a certain efinite rapidity has been 

 attained, the wire begins to glow. The colour first 

 exhibited is red, which corresponds to the lowest rate 

 of vibration of which the eye is able to take cognisance. 

 By augmenting the strength of the electric current 

 more rapid vibrations are introduced, and orange rays 

 appear. A quicker rate of vibration produces yellow, a 

 still quicker green ; and by further augmenting the 

 rapidity, we pass through blue, indigo, and violet, to the 

 extreme ultra-violet rays. 



Such are the changes recognised by the mind in the 

 wire itself, as concurrent with the visual changes taking 

 place in the eye. But what connects the wire with this 

 organ ? By what means does it send such intelligence 

 of its varying condition to the optic nerve ? Heat being 

 as defined by Locke, ' a very brisk agitation of the insen- 

 sible parts of an object,' it is readily conceivable that on 

 touching a heated body the agitation may communicate 

 itself to the adjacent nerves, and announce itself to them 

 as light or heat. But the optic nerve does not touch 

 the hot platinum, and hence the pertinence of the ques- 

 tion, By what agency are the vibrations of the wire 

 transmitted to the eye ? 



The answer to this question involves one of the most 

 important physical conceptions that the mind of man 

 has yet achieved : the conception of a medium filling 

 space and fitted mechanically for the transmission of 

 the vibrations of light and heat, as air is fitted for the 

 transmission of sound. This medium is called the 

 luminiferoua ether. Every vibration of every atom 

 of our platinum wire raises in this ether a wave, which 

 speeds through it at the rate of 186,000 miles a second- 



