DIFFRACTION. 57 



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 thickness of the interposed plate, 

 M. Arago discovered that the disappearance of the fringes in this 

 case arose from their displacement, the bands being always trans- 

 ferred to the side on which the plate was interposed. From this 

 it followed that the light was retarded in the denser medium.* 

 M. Arago afterwards produced the same modification in the inter- 

 ference bands formed by two mirrors ; and the experiment, in this 

 form, is a complete crucial instance, as applied to the two theories 

 of light. The amount of the displacement determines the velo- 

 city 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 experi- 

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

 velocity of light is inversely as the refractive index of the medium 

 traversed, f 



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 

 that the deviations produced in the different species of simple 

 light, or the distances of the bands from the axis of the pencil, 

 were in all cases proportional to the lengths of the Jits, the magni- 

 tude of the aperture remaining the same. The same analogy was 

 preserved in different media, the deviations varying in the inverse 

 ratio of the refractive indices of the media, or in the direct ratio of 

 the fits.J M. Pouillet adds, that they were unable to explain 

 these laws, having adopted the theory of emission. They are all 

 simple consequences of the wave-theory. The interval of the fits is 

 exactly half the length of a wave ; and the true connexion between 

 the place of the fringes and the latter quantity had been already 

 pointed out by Young. 



Mayer afterwards studied the phenomena of diffraction, but 

 without adding any new facts to those already known. As to the 



* "Sur un Phenomena remarquable qui s'observe dans la Diffraction de la Lu- 

 mierc." Annaletde Chimie, torn. i. 



t Phil. Jfaff., Second Series, vol. xi. p. 6. 



t Biot, Trait* de Physique, torn. iv. Supplement a 1'Optique. 



{ Siemens de Physique, torn. ii. p. 437. 



