EULOGY ON THOMAS YOUNG. 



123 



very simple laws, tlie discovery of wbicli, iu auy age, would suffice to 

 immortalize a j)bysicist. 



The differences of route wliicli produce these conflicts between the 

 rays, accompanied by their entire mutu.al destruction, have not the same 

 numerical value for the differently colored ])rimary rays. When two 

 white rays cross, it is then possible that one of their chief constituent 

 parts, the red for exami)le, may alone be iu a condition fit for mutual 

 destruction. But white deprived of its red, becomes green. Thus, in- 

 terference of light manifests itself in the phenomena of coloration. 

 Thus, the different elementary colors are placed in evidence without any 

 prism to separate them. AVe should, however, remark that there does 



this would go on alternately at successively greater thicknesses of the film, giving a 

 succession of such points or bands. 



Thus, at two successive 

 thicliucsses of the plate p, 

 the incident rays, falling 

 on it iu parallel directions 

 i i, are reflected partially 

 from the first surface r r, 

 and partially from the sec- 

 ond >■' r'. According to the 

 difference of thickness trav- 

 ersed, these may be in ac- 

 cordance giving a point of 

 brightness as -j-, or in dis- 

 cordance giving a point of 

 darkness as at o. 



If two rays or sets of waves, instead of being exactly superimposed, be supposed to 

 meet inclined at a very acute angle, in a somewhat similar way they would, at a series 

 of points, alternately conspire or clash with each other, thus giving rise to a. series of 

 bright and dark points, the assemblage of which will produce l)ands 

 or strijies on a screen intercepting the rays. Now, as to actual exper- 

 imental cases, it was in the application of this latter theoretical idea 

 that the invention of Dr. Young was i^eculiarly displayed. The 

 former case was that alone which seems to have occurred to Hooke, 

 in reference to the colors of thin plates, and even this was in his 

 mind l)ut a very indefinite conception ; nor did it seem at first sight 

 readily comparable with such cases as the diftraction fringes, or still 

 less with the internal bands of a shadow observed by Grimaldi. If 

 Hooke had imagined any theoretical views of this kind, it was j)rob- 

 ably confined to the one case of the thin films. Young's great merit 

 was the comprehensiveness of his principle ; and in following out the 

 investigation he proceeded at once to such a generalization as evinced 

 that comprehensiveness, and connected immediately those classes of 

 phenomena apparently so diflerent in character — the thin films, the 

 internal bands, and the external fringes. When, as in Grimaldi's 

 experiment, (since called the phenomena of difi'raction,) a narrow 

 slip of card was placed in a very naiTow beam of solar light, dark 

 and bright stripes iiarallel to the sides internally marked the whole 

 shadow longitudinally, while the external fringes appeared on the 

 outside at each edge. The genei'al appearance of the shadow of a 

 long narrow body with parallel sides iu a beam of solar light issu- 

 ing from a minute hole iu a shutter, or, what is better, the focus of 

 a small lens collecting the rays to a point, is that of a shadow marked 

 with longitudinal stripes and externally bordered by parallel fringes 

 or bands of light slightly colored, as seen in the annexed figure. 



To cxliibit these appearances ordinarily requires ths sun's light. 

 But the translator has found a very simple method of exhibiting 

 these phenomena on a minute scale by candle liglit.by merely placing 

 a fine wire across one surface of a lens of short focus, and looking througli it at light 

 admitted through a narrow slit parallel to the wire, or even the flame of a candle at a 

 considerable distance. 



Next, as to the theoretical explanation, an inspection of the accompanying diagram 

 will perhaps help to convey an idea of the manner in which the several sets of waves 

 are formed, and interfere in the case now supposed. 



Young couceived the beam of light as a series of waves propagated onward till, on 

 reaching the card; they were broken u}) into two new sets of waves, sjireading in circlea 



