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 5<D,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 versa.*? 



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



000,000,000. 



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

 text is only assumed for the ^avpnience of such of my readers 



