REPORT ON PHYSICAL OPTICS. 333 



disputed by M. Poisson. From his theory of the propagation 

 of motion in fluid media, this mathematician inferred that the 

 absolute velocities of the molecules are insensible in directions 

 making finite angles with the direction of the original vibra- 

 tions. He concludes, therefore, that these velocities, or the 

 intensity of the light in the partial waves, cannot be regarded 

 as sensibly equal in directions inclined to it at very small 

 angles*. Fresnel's reply to this part of M. Poisson's theory 

 has been already referred to. The principle of Huygens itself, 

 which forms the basis of Fresnel's theory, though not denied 

 by M. Poisson, is yet objected to, as introducing a needless 

 complication, into the question; and indeed it does not seem 

 easy to understand, at first view, why each point of the pri- 

 mary wave in this mode of composition should not give rise to 

 a retrograde as well as to a direct wave f. 



An objection of a different nature has been I'aised against 

 Fresnel's theory, derived from its supposed discordance with 

 phenomena. It is a consequence of that theory, when applied 

 to the case of diffraction by a narrow aperture bounded by pa- 

 rallel straight edges, that if a point be taken in the axis of the 

 pencil, whose distances measured from the centre and edge of 

 the aperture differ by half a wave, that point will be the limit 

 within which all the interior fringes are confined ; and beyond 

 that point the centre of the image will be always white. This 

 result is confirmed by the previous experiments of M. Biot, by 

 the observations of Fresnel himself, and by those of Professors 

 Airy and Powell, by whom they have been since repeated. 

 M. Biot found that the central band was dark and white alter- 

 nately, to a certain distance from the aperture ; after which it 

 was always white. He remarks that when this limit is attained, 

 we may diminish the breadth of the aperture, and even bring its 

 sides into actual contact, without any change in the central 

 band except its enlargement and consequent diminution of in- 

 tensityj. 



Newton's celebrated experiment with the two knife-edges has 



• It may be necessary to state that it was part of M. Poisson's theory, that 

 the vibrations are normal to the wave. 



t See Annales de Chimie, torn. xxii. p. 270, torn, xxiii. ; and Airy's Math. 

 Tracts, p. 267. 



I Traite de Physique, torn. iv. pp. 749, 760. The description of the pheno- 

 menon given by Mayer is very similar : " Prout ilia distantia acierum sem- 

 per magis magisque imminuitur, fasciae adeo evanescunt, ita ut denique non 

 nisi fascia media remaneat ; sed ad dextram atque sinistram adeo in latitudinem 

 extensa, ut non nisi lumen languiduifl, a medio spectri initialis utrinque instar 

 Cauda cometse sese dilatans, representet." Gotli?i(/en Memoirs, vol. iv. p. 61. 



