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 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 which science recognizes 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 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 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 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 shock of every atom of our platinum wire raises in this ether a wave, which speeds through it at the rate of 186,000