24 FRAGMENTS OF SCIENCE. 
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 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 denned 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 tiie 
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 ether undu- 
lations, 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 process 
* The action here described is analogous to the passage of sound- 
waves through thick felt whose interstices are occupied by air. 
