in Physical Optics. 59 



on the surfaces. As we have here to consider the action of 

 transparent media on waves of hght, it is necessary to make 

 suppositions respecting the constitution of such media, and the 

 dynamic relations subsisting between them and the lumiuiferous 

 aether. On these points I have ah-eady advanced, with reference 

 to a theory of physical forces, very definite hypotheses, from 

 which, in the present theory, I shall not have occasion to depart. 

 The following explanation, consequently, so far as it is success- 

 ful, may be regarded as a corroboration of the truth of the 

 hypotheses. I have supposed that visible media generally con- 

 sist of a collection of hard and inert spherical atoms, which act 

 upon each other dynamically only through the intervention of 

 undulations of the sether. Accordingly the sether is of the same 

 density within the media as without, and any effect which the 

 media produce on the undulations propagated within them is 

 due solely to the obstacles they present to the free motion of 

 the sether, and the reflexions which take place at the surfaces 

 of their atoms. In my communication to the ' Philosophical 

 Magazine,^ for last March, I have given reasons for concluding 

 that the part of the reflexions which is unaccompanied by con- 

 densation produces a mutual repulsion between neighbouring 

 atoms. But, if this be the case, it is evident that the same re- 

 flexions must also act on the fluid itself. The efl'ect of such action 

 on waves propagated in transparent media, appears to admit of 

 the following investigation. In the first place, it is to be re- 

 marked, that the breadth A, of a luminous undulation has a very 

 large ratio to the mean interval between the atoms. This fact, 

 as is known, has been established in the instance of glass, by 

 microscopic observation of finely-drawn parallel lines on its sur- 

 face, which are still seen distinctly separate, when the mean 

 interval between them is less than X. By considering, in con- 

 junction with this fact, that the radius of an atom must be ex- 

 ceedingly small compared to the interval from atom to atom, 

 some idea may be formed of the extreme minuteness of the di- 

 mensions which must be attributed to the ultimate particles of 

 matter'. Now I have elsewhere shown that if V be the velocity 

 incident on a spherical atom of radius r, supposed at rest, the 

 velocity reflected in a direction making an angle 6 with the di- 

 rection of incidence is, at the distance of R from its centre, very 



nearly equal to -^ cos 6. This expression applies to the re- 

 flexion of velocity from an atom situated apart from all others. 

 But in the case of an aggregation of atoms, the velocity reflected 

 from a given atom in its turn suffers reflexion from all the atoms 

 within a certain distance from it, and thus the dccrcnumt of 

 velocity with increase of distance takes [)]ace more rapidly than 



