RADIATION. 33 



the old vibrations the current generates new and more 

 rapid ones, and when a certain definite 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 in- 

 telligence of its varying condition to the optic nerve? 

 Heat being as denned by Locke, ' a very brisk agitation 

 of the insensible parts of an object/ it is readily con- 

 ceivable that on touching a heated body the agitation 

 may communicate itself to the adjacent nerves, and an- 

 nounce itself to them as light or heat. But the optic 

 nerve does not touch the hot platinum, and hence the 

 pertinence of the question, By what agency are the vi- 

 brations 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 

 luminiferou's 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. 

 The ether suffers no rupture of continuity at the sur- 

 face of the eye, the inter-molecular spaces of the various 



