Light from Transparent Matter. 97 



a ratio of equality. We may therefore take 



where p% is less than /jl 2 . The solution is the same as before, 

 except that now 



This explanation of the deviation of M from Green's value 

 seems to me the most probable ; but the ground might be taken 

 that the densities concerned in the propagation of the so-called 

 longitudinal waves are unknown, and may possibly not be the 

 same as those on which transverse vibrations depend, For sul- 

 phuret of arsenic, Jamm's experiments give 



/*= 2*454, /* = 1-083, 



showing that p, is very considerably less than jju. 



One of the most remarkable of Jamin's results shows that in 

 many cases M is negative, or ^ less than unity. There are a few 

 substances of an intermediate character forwhichM = 0; and then 

 FresnePs original formulae express the laws of the phenomena. 

 The value of /ul is usually about 1*45. No adequate explanation 

 has hitherto been given of the singular law ; and in the remarks 

 which follow I wish to be understood as merely throwing out a 

 suggestion which may or may not contain the germ of an ex- 

 planation. 



It is known that many solid bodies have the power of con- 

 densing gases on their surfaces, a property on which the action 

 of Grove's gas-battery seems to depend. Now, if we were to 

 suppose that at the surfaces of solid and liquid bodies there ex- 

 ists a sheet of condensed air, which need not extend to a distance 

 greater than the wave-length, but is of an optical density corre- 

 sponding to about yLt = l*5, the occurrence of negative values of 

 M would, I think, be explained. There is nothing a priori very 

 improbable in the existence of such a sheet, so far as I am able 

 to see ; but it is for experiment to decide whether the pheno- 

 mena observed near the polarizing angle depend in any manner 

 on the nature of the gas with which the reflecting body is in 

 contact, and whether the sign of M may change from negative 

 to positive when vacuum is substituted for atmospheric air. The 

 fact that the value of M for the surface of separation of (say) 

 glass and water cannot be calculated from the values of M cor- 

 responding respectively to glass and air, water and air, seems to 

 indicate that the phenomenon is, so to speak, of an accidental 

 character. 



Phil. Mag. S. 4. Vol. 42. No. 278. Aug. 1871. H 



