368 



that the force which produces reflexion, varies according to a different 

 law in different bodies. The reflective forces of the solid and the 

 fluid may be conceived to decrease in various ways : first, they may 

 respectively extend to different distances from the reflecting surface, 

 and decrease according to the same law. Secondly, they may ex- 

 tend to different distances, and vary according to a different law ; or, 

 lastly, they may extend to the same distance, and vary according to 

 different laws. Whether the refracting forces follow the same law 

 in solids and in fluids, it is extremely difficult to determine by direct 

 experiment ; but if we assume the mutual dependence of the refract- 

 ing and reflecting forces, then the experiments recorded in this paper 

 will establish a variation in the law of the refracting forces of diffe- 

 rent media. 



These facts may be explained on the undulatory theory of light, 

 by supposing that the density or elasticity of the ether varies near 

 the surface of different bodies, an hypothesis which has already af- 

 forded an explanation of the loss of part of an undulation in several 

 of the phenomena of interference ; the part lost being, according to 

 Dr. Young, a variable fraction, depending on the nature of the con- 

 tiguous media. 



The phenomena of periodical colours at the confines of media of 

 the same or of different refractive powers, are evidently dependent 

 on the law of interference, although it may be difficult to point out 

 the precise mode in which they are produced. In combinations 

 where there is much uncompensated refraction, their production is 

 influenced by certain changes, such as the formation of a thin and 

 invisible film on the surface of the solid, the nature and origin of 

 which the author endeavours to investigate, but which he acknow- 

 ledges he has hitherto been unable to discover. That some unrecog- 

 nised physical principle is the cause of all these phenomena will, he 

 thinks, appear still more probable from a paper which he intends to 

 present to the Society, on the production of the very same periods of 

 colour, at similar angles of incidence, by the surfaces of metals and 

 transparent solids, when acting singly upon light. He also an- 

 nounces, as the subjects of two other communications, the results of 

 researches in which he has been long engaged ; first, on the action of 

 light on the surfaces of bodies as an universal mineralogical character, 

 with the description of a lithoscope for discriminating minerals ; and 

 secondly, on the influence of the doubly refracting forces upon the ordi- 

 nary forces, which reflect and polarize light at the surfaces of bodies. 



On the Reduction to a Vacuum of the Vibrations of an Invariable Pen- 

 dulum. By Captain Edward Sabine, of the Royal Artillery, Scc.R.S. 

 Communicated by Dr. Thomas Young, Secretary of the late Board 

 of Longitude. Read March 12 and 19, 1829. [Phil. Trans. 1829, 

 p. 207.] 



The experiments contained in this paper originated in a no- 

 tice published by M. Bessel in the Astronomische Nachrichten, for 



