304 TH03IAS YOUNG. 



Two rays, proceeding from the same source by 

 slightly unequal routes, crossed one another at a cer- 



own words: " It was in May, 1801, that I discovered by reflecting on 

 the beautiful experiments of Xewton, a law which appears to me to 

 account for a greater variety of interesting phenomena than any other 

 optical principle that has yet been made known. I shall endeavour 

 to explain this law by a comparison: Suppose a number of equal 

 waves of water to move upon the surface of a stagnant lake, with a 

 certain constant velocity, and to enter a narrow channel leading out 

 of the lake; suppose, then, another similar cause to have excited 

 another equal series of waves, which arrive at the same channel with 

 the same velocity, and at the same time with the first. Neither series 

 of waves will destroy the other, but their effects will be combined; if 

 they enter the channel in such a manner that the elevations of the one 

 series coincide with those of the other, they must together produce a 

 series of greater joint elevations; but if the elevations of one series 

 are so situated as to correspond to the depressions of the other, they 

 must exactly fill up those depressions, and the surface of the water 

 must remain smooth; at least, I can discover no alternative, either 

 from theory or from experiment. Xow, I maintain that similar effects 

 take place whenever two portions of light are thus mixed; and this I 

 call the general law of the interference of light." * Translator. 



For the sake of many readers, it may not be superfluous or useless 

 here briefly to illustrate the application of these theoretical ideas. 

 We have only to imagine in like manner, in the case of the rays of 

 light, two sets of waves propagated thi*ough an ethereal medium and 

 coinciding in direction, when it will be easily apparent that just as in 

 the case of the supposed canal, they may have their waves either 

 conspiring or counteracting, and consequently giving a point of bright- 

 ness or darkness accordingly. 



Thus, a coincidence in the periods, or an interval of an integer num- 

 ber of entire wave-lengths, would cause the two systems of waves to 

 conspire and reinforce each other; a difference of periods of half a 

 wave-length, or any odd number of half wave-lengths, would cause 

 the two systems to counteract or neutralize each other. Thus, 

 according to the thickness, there would be a point of darkness or of 

 brightness for each primary ray, and the succession of tints would 

 be perfectly explained. 



This would directly apply to the thin films. A ray impinging would 

 be partly reflected at the first surface of the thin film, partly entering 



i Works, vol. i. p. 202. 



