172 FRAGMENTS OF SCIENCE. 



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 insensible 

 parts of an object," it is readily conceivable that on touch- 

 ing 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 plati- 

 num, 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, perhaps, the most 

 important physical conception 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 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 com- 

 munication 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 the analysis 

 of science. As an oar dipping into the Cam generates 

 systems of waves, which, speeding from the centre of dis- 

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

 the vibrating atoms generate in the surrounding ether un- 

 dulations, which finally stir the filaments of the retina. 

 The motion thus imparted is transmitted with measurable 

 and not very great velocity to the brain, where, by a pro- 



