24 PR A GMENTS OF SCIENCE. 



quicker rate of vibration produces yellow, a stillquicker, 

 green; and by further augmenting th-e rapidity, we pass 

 through blue, indigo, and violet, to the extreme ultra- 

 violet rays. 



Such are the changes recognized 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 vary- 

 ing condition to the optic nerve? Heat being as defined by 

 Locke, "a very brisk agitation of the insensible 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 question, 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 vibra- 

 tions of light and heat, as air is fitted for the transmission 

 of sound. This medium is called the luminiferous 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 surface of the eye, the inter-molecular 

 spaces of the various humors are filled with it; hence the 

 waves generated by the glowing platinum can cross these 

 humors and impinge on the optic nerve at the back of the 

 eye.* Thus the sensation of light reduces itself to the 

 acceptance of motion. Up to this point we deal with pure 

 mechanics ; but the subsequent translation of the shock of 

 the ethereal waves into consciousness eludes mechanical 

 science. As an oar dipping into the Cam generates sys- 

 tems of waves, which, speeding from the center of disturb- 

 ance, finally stir the sedges on the river's bank, so do the 

 vibrating atoms generate in the surrounding ethur undu- 

 lations, which finally stir the filaments of the retina. The 

 motion thus imparted is transmitted with measurable, ;md 

 not very great velocity to the brain, where, by a process 



* The action here described is analogous to the passage of sound- 

 waves through thick felt whose interstices are occupied by air. 



