26 Mr. G. A. Schott. On the [Jan. 18, 



as obtained by the platinum thermometers are independent of the 

 ratio of the bridge arms, and are, therefore, unaffected. The values 

 of the temperature coefficients, of the capacity for heat of water and 

 of the specific heat of the calorimeter, remain practically unaltered, 

 as the correction only affects the sixth significant figure. 



The corrected value of J in terms of a thermal unit at 15 C. is 

 thus (4*1940 + 0-0042) x 10 7 = 4-1982 xlO 7 , and I estimate the 

 limit of error due to the causes mentioned in Section I of this 

 Appendix as 0-0020. 



Hence (assuming that g = 981*17), 



J = 427'88 kilogramme-metres in latitude of Greenwich. 



J = 1403'6 ft.-lbs. per thermal unit C. in latitude of Greenwich. 



J = 779-77 F 



V. " On the Reflection and Refraction of Light." By G. A. 

 SCHOTT, B.A. (Cantab.), B.Sc. (Lond.), formerly Scholar of 

 Trinity College, Cambridge. Communicated by R. T. 

 GLAZEBROOK, M.A., F.R.S. Received November 29. 1893. 



(Abstract.) 



The object of this paper is to examine the consequences of suppos- 

 ing the transition between different refractive media to be effected 

 continuously through a thin variable layer, to deduce expressions for 

 the amplitudes and changes of phase of the reflected and refracted 

 light, and to compare them with the results of experiments hitherto 

 made on that subject. 



The theories examined are the elastic solid theories, both those 

 assuming large velocities for the pressural wave, including Green's, 

 Voigt's, and K. Pearson's theories, and also Lord Kelvin's contractile 

 ether theory, and then the electromagnetic theory, in the form given 

 by Hertz, which, it may be remarked, leads to the same equations as 

 the contractile ether theory. 



The medium being continuously variable, the displacements and 

 stresses, or the electric and magnetic force components, are every- 

 where continuous. The method thus avoids all hypotheses as to 

 boundary conditions at surfaces of discontinuity. 



For convenience, the first constant portion of the medium, from which 

 the incidental light comes, is called the first medium, the second con- 

 stant part, into which the light is in part refracted, is called the second 

 medium, the thin variable part is called the variable layer, and the 

 arbitrarily chosen planes, which include the whole of the variable 

 layer, are called the boundaries of the layer. Since at those planes 

 the medium is continuous, the displacements and stresses have the 

 same values on both sides of each plane. 



