304 THOMAS 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 Newton, 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. Now, 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 through 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 
1 Works, vol. i. p. 202. 
