Astronomical and Nautical Collections. 445 



therefore no longer be in the bright stripe. The stripe must 

 therefore necessarily change its place towards the pencil which 

 travels the more slowly, in order that the shortness of its 

 path may compeiiisate for the delay during its transmission 

 through the solid : and the converse of the proposition 

 enables us to conclude, that where the stripes are displaced, 

 the pencil towards which they move has been retarded in its 

 passage. The natural, inference, therefore, ** fromMr. Arago*s 

 experiment," is, that light is propagated more rapidly iu 

 the air than in mica or glass, and generally in all bodies more 

 refractive than the air ; a result directly opposite to the 

 Newtonian theory of refraction, which supposes the particles 

 of light to be strongly attracted by dense substances, which 

 would cause the velocity of light to be greater in these bodies 

 than in rarer mediums. 



This experiment furnishes a method of comparing the 

 velocity of the propagation of light in different mediums, [or, 

 in other words, the refractive density, which is always sup- 

 posed in this theory, to be reciprocally proportional to it.] 

 If, in fact, we measure very accurately, by means of a sphe- 

 rometer, the thickness of the thin plate of glass which has 

 been placed in the way of one of the luminous pencils, and 

 if the displacement of the fringes has been measured by the 

 micrometer ; since we know that, before the interposition of 

 the glass, the paths described were equal for the middle of 

 the central stripe, we may calculate how much difference is 

 occasioned by the change of position, and this difference 

 will give the retardation in the plate of glass, of which the 

 thickness is known : so that, by adding this thickness to the 

 difference calculated, we shall find the little path which the 

 other pencil has described in the air, while the former was 

 transmitted by the plate of glass; and this path, compared 

 with the thickness of the plate of glass, will give the propor- 

 tion of the velocity of the light in the air, to its velocity 

 within the glass. 



We may also consider this problem in another point of 

 view, with which it is convenient to make ourselves familiar. 

 The duration of each undulation, as we have seen, does not 

 depend on the greater or less velocity with which the agita^ 



OCT.— DEC. 1827. 2 G 



