122 EULOGY ON THOMAS YOUNG. 



Two rays proceeding from tlie same source by slightly uueqnal routes, 

 crossed one anotlier at a certain point iu space. At this point was 

 placed a slieet of white paper. Eacli ray, taken by itself, made the 

 l)aper more bright at that point, but when the two rays united and 

 arrived at that point together all brightness disappeared 5 complete 

 night succeeded to day. 



Two rays do not always annihilate each other completely at their 

 point of intersection. Sometimes we observe only a partial weakening 

 of intensity ; sometimes, on the other hand, the rays conspire and in- 

 crease the illumination. Everything depends on the diflference in the 

 length of route w^hich they have gone through, and that according to 



! 



tions had, iudeecl. been put forth respecting a theory of emitted molecules on the one 

 hand, and of waves in an ethereal medium on the other, and a few experimental facta 

 bearing on the choice between such hypotheses had been ascertained. 



The several distinct j)lieuomena of common reflection and refraction, of donble 

 refraction, of inflection or diffraction, and of tlie colored rings did not seem to be con- 

 nected by any common principle, nor, even separately considered, could it be said that 

 they were very satisfactorily ex])lained. It was now the peculiar distinction of Young 

 to perceive, and to establish in the most incontestable manner, a great principle of the 

 simplest kind, which at once rendered the wave hypothesis applicable to the two last- 

 named classes of facts, and thus directly connected them with the former. It is not 

 always that we are enabled to trace the iirst rise and progress of the idea of a great 

 discovery in the inventor's mind. We cannot forbear from here noticing that Dr. 

 Young has left on record the progress of the first suggestions which occurred to him 

 on the subject of interference. The first view which juesented itself was that of the 

 analogies furnished by sound, which, as is well known, is conveyed by means of waves 

 proi^agated in air; and iu the case of two sounds diflering a very little from the same 

 pitch, produced at the same time, we have not a continuous sound, but heats — that is, 

 alternations of sound and silence; the waves iu the one case conspiring with and reen- 

 forciug each other, iu the other counteracting, neutralizing, and destroying each other. 

 But in more special reference to light. Dr. Y'oung's account of the origin of his ideas is 

 so clear and striking that w« nnist give it in his own words : " It was in May, 1801, that 

 I discovered, by rdiectiug 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 endeavor 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 efi'ects 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 cor- 

 respond 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 eliccts take 

 place whenever two portions of light are thus mixed, and this I call the general law of 

 the interference of light." — Tkanslatoe. 



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 propagatcnl through an ethe- 

 real medium and coinciding iu direction, when it will be easily apparent itbat just as 

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

 counteracting, and consequently giving a point of brightness or darkness accordingly. 

 Thus, a coiucidence in the periods, or an interval of an integer imniber of entire wave- 

 lengths would cause the two systems of waves to conspire and reenforce each other ; 

 a difference of periods of half a wave-length, or any odd number of Lalf wave-lengths, 

 would cause tlie two systems to counteract or neutralize each other. Thus, according 

 to the thickness, there would bo a point of darkness or of brightness for each primary 

 ray, and the succession of tints would be perfectly explained. 



This would directly apjily to the //( in films. A ray impinging would be partly reflected 

 at the first surface of the thin film; partly entering, it would be reflected internally at 

 its second surface, and emerge coincidiug in f/(rcc//o« with the first, but retarded behind 

 it from the thickness traversed in its nnd illations either by a whole or half undulation, 

 or some multiples of these, thus giving either a point of brightness or one of darkness 

 accordingly; or by some intermediate fraction giving an intermediate shade. Aild 



