242 



Sir J. Conroy on 



[Jan. 9, 



of the above range, more especially for any of which the period is 

 between 24 and 25 days. We find strong evidence of an inequality of 

 considerable magnitude of which the period is 24*00 days, very nearly. 

 We have also found preliminary evidence of the existence of two 

 considerable inequalites of periods not very far from 24*65 and 

 24*80 days. These two appear to come together in about 11 years, 

 but we cannot yet give the exact time of this. 



We have not found a trace of any inequality with a period of 

 24*25 days. 



V. " Some Experiments on Metallic Reflexion." By Sir John 

 Conroy, Bart., M.A. Communicated by Professor G. G. 

 Stokes, Sec. R.S. Received November 18, 1878. 



In the experiments made by Sir David Brewster, M. Jamin, Pro- 

 fessor Haughton, and others, on the light reflected by polished metallic 

 surfaces, the reflecting surfaces were in contact with air; and, as far 

 as I am aware, the only observations which ha^e been made when the 

 reflecting surfaces were in contact with other media are those by 

 Quincke, an account of which is given in " Poggendorff's Annalen," 

 vol. cxxviii, p. 541, and in the " Jubelband," p. 336. He found that 

 he obtained different values for the principal incidence and prin- 

 cipal azimuth, according as the reflecting surface of a film of silver 

 was in contact with air, crown glass, flint glass, water, or turpentine, 

 and that the only connexion between the values of these angles and 

 the refractive index of the medium in which the reflexion took place 

 was, that in general with the same metal, the principal incidence and 

 the principal azimuth became less as the refractive index of the 

 medium increased. 



I therefore hope that a short account of some attempts that I have 

 recently made to determine the principal incidence for, and the prin- 

 cipal azimuth of, the light reflected by polished surfaces of gold and 

 copper in contact with different media, may be of interest. 



The experiments are, I regret to say, incomplete, as, finding that my 

 eyes were beginning to suffer, I thought it best, for the present at 

 least, to discontinue them. 



I used a Babinet's goniometer, to the arms of which two tubes con- 

 taining nicols were attached, a vertical divided circle being fixed at 

 one end of each, so that the position of the nicols could be read by a 

 vernier to 5'. The goniometer had, in addition to the horizontal 

 stage, a vertical one, so arranged that the reflecting surface could be 

 placed in the axis of the instrument ; toothed wheels, working into a 

 pinion rotating on an axis fixed in one of the arms of the divided 

 circle, were attached to the vertical stage, the position of which could 



