CHAPTEE VI. 



* 

 INTERFERENCE OF LIGHT. 



(98) HAVING considered the mode of propagation of a 

 luminous wave, and the modifications which it undergoes on 

 encountering the surface of a new medium, we may now pro- 

 ceed to inquire what will be the effect, when two series of 

 waves are propagated simultaneously from two near luminous 

 origins. 



It is obvious that when two waves one proceeding from 

 each source arrive at any instant at the same point of space, 

 the particle of ether there will be thrown into vibration by 

 both ; and we are to consider what will be the result of this 

 compound vibration. Now, it is demonstrated by analysis, 

 that when two small vibrations are communicated at the same 

 time to a material point, each of them will subsist indepen- 

 dently of the other ; and the motion of the point will, in 

 consequence, be the resultant of the motions due to each vi- 

 bration considered separately. This principle is denominated 

 the superposition of small motions. Its nature may be made 

 clear by a simple instance. 



Let a pendulous body receive an impulse in any plane 

 passing through the point of suspension : it will then, of 

 course, vibrate in that plane. Now, at the lowest point of 

 the arc of vibration, let a second impulse be given to the 

 moving body, in a direction perpendicular to the plane in 

 which it already vibrates. This impulse, if communicated to 

 the 'body at rest, would cause it to vibrate in a plane at right 

 angles to the former, and through an arc depending on the 

 magnitude of the impulse. Now it will be found, on trial, 



