ON LIGHT. 315 



carried from its point of repose, or its medium situation, 

 in the act of vibration; the acting or coercive force 

 must suffice to bring it back from that distance in one 

 fourth part of that inconceivably minute fraction of a 

 second by which, as above shown, the period of a com- 

 plete vibration is expressed. Taking the case, then, of 

 any particular ray (as for instance that between the 

 green and blue rays of the spectrum, corresponding 

 to a wave-length of one 5o,oooth of an inch, and to a 

 period of one 589 billionth of a second), if we assume 

 the extent of excursion, we can very readily calculate the 

 intensity of the force (as compared with that of gravi- 

 tation) which, acting uniformly during that time, would 

 urge it through that space. Let us suppose then, that the 

 nerves of the retina are so constituted as to be sensibly 

 affected by a vibratory movement of no greater extent 

 or amplitude than one trillionth* part of an inch either 

 way; and the calculation executed, we shall find that 

 a force exceeding that of gravity in the proportion of 

 nearly thirty thousand millions to one must be called 

 into action to keep up such a movement. Our choice 

 lies between two immensities, we had almost said be- 

 tween two infinities. If we would bring the force within 

 the limits of human comprehension, we must in the 

 same proportion exaggerate the delicacy of our nervous 

 mechanism, and vice 



* A trillion is a million of billions = io 18 , or 1,000,000,000,- 



000,000,000. 



t The hypothesis of a uniform action of the coercive force in the 

 text is only assumed for the convenience of such of my readers 



