RADIATION 39 



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 ac- 

 ceptance 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 centre of dis- 

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

 do the vibrating atoms generate in the surrounding ether 

 undulations, 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 proc- 

 ess which the science of mechanics does not even tend to 

 unravel, the tremor of the nervous matter is converted into 

 the conscious impression of light. 



Darkness might then be defined as ether at rest; light 

 as ether in motion. But in reality the ether is never at 

 rest, for in the absence of light-waves we have heat-waves 

 always speeding through it. In the spaces of the universe 

 both classes of undulations incessantly commingle. Here 

 the waves issuing from uncounted centres cross, coincide, 

 oppose, and pass through each other, without confusion or 



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

 through thick felt whose interstices are occupied by air. 



