36 REPORT ON PHYSICAL OPTICS. 



dense bodies to be the body itself, the ether within it, or both con- 

 jointly. Euler maintained the first of these opinions, and believed 

 that light was propagated through the gross particles alone, in the 

 same manner as sound. But this hypothesis is contradicted by 

 the most obvious facts ; and according to it, as Dr. Young has 

 observed, the refraction of the rays of light in our atmosphere 

 should be a million times greater than it is. Of the other two 

 opinions, Young seems to have held the latter, and to have 

 thought that the molecules of the body formed, together with 

 those of the ether within it, a compound vibrating medium, which 

 was denser than the ether alone, but not more elastic. Others, 

 lastly, attribute the propagation of light in transparent bodies to- 

 the vibrations of the ether alone, that fluid being retained by the 

 attraction of the body in a state of greater density within it than 

 in free space. 



A very different view of this subject has been recently main- 

 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 aberration this 

 mathematician conceives that the reflexion of light, and its re- 

 tardation 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.* 

 lie 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 r 

 without change of density. t The former of these, he thinks, will 

 account for the reflexion of light, the latter for the diminished 

 velocity of transmission.^ This ingenious theory has the advan- 



* 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 Phil Mag., New Series, vol. xi. 



I 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 



M 3 - 1 = tf X; 



in which 5 denotes the density of the medium, and H a constant proportional to 

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



