88 FRAGMENTS OF SCIENCE 



swing through wider ranges. Technically speaking, the 

 amplitudes of the oscillations are increased. The current 

 does this, however, without altering the periods of the old 

 vibrations, or the times in which they were executed. But 

 besides intensifying the old vibrations the current gener- 

 ates 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 low- 

 est rate of vibration of which the eye is able to take cog- 

 nizance. By augmenting the strength of the electric cur- 

 rent 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 rapid- 

 ity, we pass through blue, indigo, and violet, to the ex- 

 treme 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 varying condition to the optic nerve? Heat being, as 

 defined by Locke, "a very brisk agitation of the insen- 

 sible 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 vibrations 



