1879.] some Experiments on Metallic Reflexion. 243 



be read by a vernier to 15", and to the telescope arm ; the ratio of the 

 wheels to the pinion being such that, on moving the telescope arm, 

 the vertical stage also moved in the same direction, but with half the 

 angular velocity ; so that when the reflecting surface had been pro- 

 perly adjusted, the light which passed along the axis of the tube fixed 

 to the collimator arm, was reflected along the axis of that fixed to the 

 telescope arm, at all angles of incidence. 



A quarter undulation plate was placed at the inner end of the tube 

 fixed to the collimator arm, and a small direct vision spectroscope, 

 with a photographic scale, could be attached to the other tube. The 

 lower edge of the vertical stage being some distance above the gra- 

 duated circle of the goniometer, a cylindrical vessel of thin glass, 

 about 6 centims. in diameter, could be placed on the horizontal stage, 

 so as to surround the lower part of the reflecting surface ; this being 

 filled with the liquid, and a narrow vertical slit placed so as to limit 

 the incident light, fairly good observations cquld be made when the 

 reflecting surface was in contact with various liquids. 



When the principal section of the first or polarizing nicol was in- 

 clined at an angle of 45° to the plane of incidence, and one of the 

 neutral axes of the quarter undulation plate placed in that plane, 

 the transmitted light was elliptically polarized ; and at a particular 

 incidence, varying with its refrangibility, it was reflected by the metallic 

 surface as plane polarized light ; the plane of polarization being de- 

 termined by the second nicol. 



Had the retarding plate really been "a quarter undulation plate " 

 for light of any given wave-length, the angle of incidence at which it 

 was reflected as plane polarized light, and the azimuth of its plane of 

 polarization, would have been , the principal incidence and principal 

 azimuth for light of that refrangibility. 



The retardation of a given plate varies so much for different por- 

 tions of the spectrum, that even had it been possible to obtain one 

 producing a retardation of exactly 90° for light of any definite re- 

 frangibility, it would have differed greatly from a quarter plate for 

 other portions of the spectrum. 



Both the neutral axes of the plate were successively placed in the 

 plane of incidence, and the mean of the two values of the angle of 

 incidence taken as the principal incidence. 



This arrangement is very similar to the one used by Dr. Eilhard 

 Wiedemann in his observations on the light reflected by surfaces of 

 fuchsine and copper, and described in " Pogg. Ann.," vol. cli, p. 6. 

 In Dr. Wiedemann's experiments the angle of incidence remained 

 constant, the position of the quarter undulation plate and of the aiicol 

 being varied; whilst in mine, the position of the quarter undulation 

 plate was constant, and the angle of incidence and the position of the 

 nicol were altered. By this means the principal incidence and azimuth 



