REPORT ON PHYSICAL OPTICS. G29 



the distance at which the fringes are received, or the other con- 

 ditions of the experiment. And, finally, comparing these con- 

 stants with the similar intervals of the two pencils reflected by 

 the surfaces of a thin plate, as deduced from the experiments 

 of Newton, he found that their difference was within the limits 

 of error to which such observations are liable, and that we are 

 warranted in concluding that the two classes of phenomena are 

 to be referred to one simple principle*. It is true, that in 

 these calcidations, Young starts from an erroneous principle re- 

 specting the lights which form the diffracted fringes by their 

 interference, and he has remarked some discordances in his re- 

 sults which have, no doubt, their origin in that circumstance ; 

 but the results of the exact theory are not greatly different from 

 that which he adopted, and the more complete analysis of Fresnel 

 has only tended to confirm the conclusion obtained by Young. 



The important experiment of Young, on the disappearance 

 of the fringes in the shadow of a narrow opaque body, when 

 the light passing by one of its edges was intercepted, was that 

 which first led him to the principle of interference. An in- 

 structive variation in this experiment was made by M. Arago. 

 The interior fringes were found to disappear likewise when the 

 light passing by one of the edges was transmitted through a 

 plate of some transparent substance ; and by varying the 

 thickjiess of the interposed plate, M. Arago discovered that the 

 disappearance of the fringes in this case arose from their dis- 

 place?nent, the bands being always transferred to the side on 

 which the plate was interposed. From this it followed, that the 

 light was retarded in the denser medium f. M. Arago afterwards 

 produced the same modification in the interference bands formed 

 by two mirrors ; and the experiment, in this form, is a com- 

 plete crucial instance, as applied to the two theories of light. 

 The amount of the displacement determines the velocity of 

 light in the medium, and therefore the refractive index, with an 

 accuracy unattainable by any other method. Professor Powell 

 has suggested a very elegant modification of this experiment, 

 which at once establishes the truth of the law, that the velocity 

 of light is inversely as the refractive index of the medium tra- 

 versed J. 



The experimental laws of the diffracted fringes were next 

 examined by MM. Biot and Pouillet. In the case of a narrow 

 rectilinear aperture, (which was that chiefly studied,) they found 



* " Experiments and Calculations relative to Physical Optics," Phil. Trans. 

 t " Sur un Phenomene remarquable qui s'observe dans la Diffraction de la 

 Lumiere," Annales de C/iimic, torn. i. 

 J P/til. May., Second Series, vol. xi. p. 6. 



