294 RADIATION. 



What is the nature of these changes ? In virtne of what condition does the 

 wire radiate at all f We tnnst now look from the wire as a whole to its con- 

 stituent atoms. Could we see those atoms, even before the electric current has 

 begun to act upon them, we should find them in a state of vibration. In this 

 vibration indeed consists such warmth as the wire then possesses. Locke enun- 

 ciated this idea with great precision, and it seems placed beyond the pale of 

 douV)t by the excellent quantitative researches of Mr. Joule. "Heat," says 

 Locke, '■'■ is a very brisk agitation of the insensible parts of the object, which 

 produce in us that sensation from which we denominate the object hot ; so what 

 in our sensation is heat in the object is nothing but motion^ When the electric 

 current, still feeble, begins to pass through the wire, its first act is to intensify 

 the vibrations already existing, by causing the atoms to sv.-ing through wider 

 ranges. Technically speaking, the amplitudes of the oscillations are increased. 

 The cuiTcnt does this, however, without altering the i)eriad of the old vibra- 

 tions, or the time in which they were accomplished. But, besides intensifying 

 the old vibrations, the current generates new and more rapid ones, and when a 

 certain definite rapidity has been attained the wire begins to glow. The color 

 first exhibited is red, which corresponds to the lowest rate of vibration of which 

 the eye is able to take cognizance. By augmenting the strength of the electric 

 current, more rapid vibrations are introduced, and orange rays appear. A 

 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 which science recognizes in the wu'e itself, as concur- 

 rent 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 f Heat being, as defined by Locke, " a 

 very brisk agitation of the insensible parts of an object," it is readily conceiv- 

 able that on tottchhig 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 viVirations 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 luminifcrous ether. Every shock of every atom of our ]>latiniun 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 tiie 

 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 communication 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 disturbance, 

 finally stir the sedges on the river's bank, so do "the viVn-ating 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 process which science does not even tend to 

 unravel, the tremor of the nervous matter is converted into the conscious impres- 

 sion 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 incessaijtlv conmiingle. Here the waves issuing 



