ON LIGHT. 313 



ear could appretiate vibrations of this degree of fre- 

 quency, the sensation corresponding to the middle ray 

 of the spectrum would be that of a note about forty- 

 two octaves above the middle C of a pianoforte. 



(98.) In each of these inconceivably minute intervals 

 of time (compared to which a single second is a sort of 

 eternity), a process has been gone through by every molecule 

 of the ether concerned in the propagation of the ray: a pro- 

 cess as strictly definite, as exactly regulated, as the 

 movement of a drop of the ocean in its conveyance of 

 the tide-wave. Taking up its motion from the particle 

 immediately behind it (whose movement it exactly 

 imitates), and transmitting it on to that immediately 

 before, it starts from rest, not suddenly, with a jerk, but 

 (under the strict control of those elastic forces already 

 mentioned), increasing gradually in speed to a maxi- 

 mum then, as gradually, relaxing, coming to a momen- 

 tary rest, and retreating to its original position by the 

 same series of measured gradations in reverse order, to 

 be ready in its place for the reception of the next im- 

 pulse. Nor does it seem possible to avoid the conclu- 

 sion, if we trace up the movement to its commencement 

 to the ' source of the light the material particle in 

 whose combustion or incandescence it originates that 

 such is the actual vibratory movement of that particle 

 itself. And thus we are brought into the presence of 

 the working of that mechanism by which flame and in- 

 candescence (" pXoywfl-ov ffv}{jt,a Kvgos the brilliant miracle 

 of fire," as the Greek poet* not inaptly terms it) are pro- 

 * ^Eschylus. 



