REPORT ON PHYSICAL OPTICS. 311 



tained by Mr. Challis. Assuming that the density of the ether 

 is the same in solid media as in free space, (an assumption 

 which he seems to think required by the phenomenon of aberra- 

 tion,) this mathematician conceives that the reflexion of light, 

 and its retardation in the denser medium, may be both accounted 

 for by the reflexions which the ethereal waves undergo from 

 the solid particles of the medium which they encounter in their 

 progress *. He shows, in fact, that the absolute velocities 

 impressed upon the ethereal particles by such reflexion may be 

 resolved into two parts, one of which is propagated uniformly, 

 and is accompanied by a change of density ; while the other is 

 propagated instantaneously, without change of density f. The 

 former of these, he thinks, will account for the reflexion of light, 

 the latter for the diminished velocity of transmission J. This 

 ingenious theory has the advantage of connecting the velocity of 

 propagation in dense bodies directly with their constitution, and 

 so of advancing a step in the process of physical induction. On 

 the other hand, it requires us to admit that the particles of ether 

 and those of gross bodies exert no mutual action of any kind. 

 We know too little of the ether, or of its properties, to deny this, 

 simply because it is unsupported by any of the properties of 

 matter hitherto revealed ; but it must at the same time be ad- 

 mitted that the violation of such analogies furnishes an argument 

 of some weight against the theory which demands them. 



Whatever supposition we may frame respecting the constitu- 

 tion of bodies, or of the ether within them, in the wave-theory, 

 it must be such that the velocity of propagation is less in the 

 denser medium. In the theory of emission, on the other hand, 

 it is the reverse ; so that although it conducts to the same result, 

 it does so by an opposite route. Here, then, the rival theories are 

 at issue upon a matter of fact • and we have only to ascertain 



• This manner of conceiving the reflexion of light, in the wave-theory, was 

 that originally entertained by Fresnel, and was put forward in a memoir read 

 to the French Academy in 1819. 



t P/iii- Mag., New Series, vol. xi. 



X The mean effect of these reflexions, Mr. Challis shows, is equivalent to 

 that of a retarding force ; and, by a certain supposition respecting its value, 

 he has arrived at the following simple formula for the determination of the ratio 

 of the velocities of propagation in free space and in the medium 



^2 — 1 = ^SH; 

 in which S denotes the density of the medium, and H a constant proportional 

 to the mean retarding effect of a given number of its molecules. For the gases, 



then, the quantity — t is nearly constant, whatever be the compression. 



This result is a very simple consequence of the theory of emission ; its ex- 

 perimental truth has been established by MM. Biot and Arago. Phil Mug., New 

 Series, vol. vii. 



