72 REPORT ON PHYSICAL OPTICS. 



principle of interference was discovered by Young. When this 

 principle was combined with the suggestion of Hooke, the whole 

 mystery vanished. The application was made by Young himself; 

 and all the principal laws of the reflected rings were readily and 

 simply explained, by the interference of the two portions of light 

 which are reflected at the two surfaces of the plate.* In applying 

 this principle, however, Young perceived that the interval of re- 

 tardation was not simply that due to the difference of the paths 

 traversed by the two pencils ; but that one of them must be sup- 

 posed to undergo a change of phase, amounting to half an undula- 

 tion, at the instant of reflexion. Young clearly pointed out the 

 accordance of this effect with mechanical principles ; and the con- 

 nexion has been fully confirmed by the more complete investiga- 

 tions of Fresnel. In fact, the two reflexions take place under 

 opposite circumstances, one of the portions being reflected at the 

 surface of a rarer, and the other at that of a denser, medium ; and 

 the laws of impact of elastic bodies indicate that the direction of 

 the vibratory movement must be reversed by reflexion in the one 

 case, while in the other it is unchanged. Young had the satis- 

 faction of putting this principle to the test in a remarkable man- 

 ner. It followed from it that, if the thin plate were of a refractive 

 density intermediate to those of the two media within which it 

 was inclosed, the laws of the phenomenon would be determined 

 by the difference of the paths alone, the reflexion being of the 

 same kind at the two surfaces. Young accordingly predicted that 

 in this case the rings should commence from a white centre, instead 

 of a black one, and the prediction was soon after verified on trial, f 

 The transmitted rings are accounted for, in the wave-theory, 

 by the interference of the direct light with that which has under- 

 gone two reflexions within the plate ; and it follows from the pre- 

 ceding considerations that their colours must be complementary 

 to those of the reflected system. This origin at once shows the 

 reason of the fact observed by M. Arago, that the light of the 

 transmitted rings is polarized in the plane of reflexion. M. Biot 

 has laboured to reconcile this fact to the theory of emission, with 

 which it appears, at first view, at utter variance. The account 

 which he has given of the phenomenon will, I think, be hardly 

 deemed satisfactory.? 



^ On the Theory of Light and Colours." Phil. Trans. 1802. 

 h "Account of some Cases of the Production of Colours." .PM. Trans. 1802. 

 t See Biot's " TraiU de Physique," torn. iv. pp. 308, et *eq. 



