250 On some Experimants on Metallic Reflexion. [Jan. 16, 



Sir David Brewster. / Q / 



(" Optics," ed. 1853, p. 309, 311). . 70 45 .. 33 for jewel- 

 Professor Haughton. lers' gold. 



(" Phil. Trans.," 1863, p. 81) 75 37 . . 47 47 



G. Quincke. 



(" Pogg. Jnbelband," p. 336) 72 47 . . 43 12 for C line. 



Assuming that the tangent of the angle of principal incidence is 

 the index of refraction of the metal for red light, the value of that 

 angle in air, as deduced from the measurements made in water and 

 carbon bisulphide with the quarter undulation plates, is 76*53 and 

 77-22 instead of 76°. 



The numbers given by Quincke (" Pogg. Ann.," vol. cxxviii, p. 541) 

 for silver are — 



Principal incidence. Principal azimuth. 



In air 74 19 43 48 



In water 71 28 44 03 



In turpentine 69 16 43 21 



The value for the principal incidence in air calculated according to 

 the same assumption, by multiplying the tangent of the principal 

 incidences in water and turpentine by the refractive indices of these 

 substances, is 75° 55' and 75° 36' instead of 74° 19' ; in all four cases 

 the value is too high. 



Although more experiments are required to decide this point, it 

 seems probable that this relationship between these numbers is not 

 merely an accidental one ; and if so that there is additional reason for 

 adhering to Sir David Brewster's opinion that the value of the angle 

 of principal incidence may be taken as indicating the refractive power 

 of a metal. 



In conclusion, I must express my thanks to Professor Stokes for 

 much advice and assistance, and specially for pointing out the method 

 for determining the residual corrections to the results obtained with 

 the quarter undulation plates. 



January 16, 1879. 



W. SPOTTISWOODE, M.A., D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



