464 On the Reflexion of Light from Iceland Spar. [Dec. 16, 



were taken that the polishing was effected under exactly the same 

 conditions. To ensure that the face was polished parallel to itself, 

 the inclinations of the face to fixed faces were measured before and 

 after polishing. The mean of about 400 readings was taken with 

 each state of the face of the crystal, to obtain as accurate a result as 

 possible. 



A second series of experiments was instituted with a view to 

 determine the accuracy with which the means of each of the previous 

 sets of observations could be determined. At the same time, a simple 

 analyser, consisting of a Nicol and a graduated circle, was set up 

 in addition to the elliptic analyser, with the object of testing the 

 conclusions of Sir John Conroy as regards polished surfaces, which are 

 given in the 'Proceedings of the Royal Society ' for February, 1886. 

 The effect of rotation of the face of the crystal through a small 

 angle in its own plane is also discussed. The observations are 

 divided into sets, the means of which are compared with each 

 other. 



Lastly, a set of experiments was made with a cleavage face split 

 off near the former, and the results of these experiments were com- 

 pared with those of the first series made some fifteen months 

 previously. 



The results of the experiments are embodied in tables, from which 

 the following numerical results are extracted. I' in the case of the 

 simple analyser corresponds to I for the elliptic analyser. 



The general conclusions of the paper are as follows : — 



The process of polishing the surface of a crystal of Iceland spar 

 with emery and rouge does most certainly alter the state of the 

 surface. This alteration is evinced by a change both in the ratio of 

 the axes and in the azimuth of the major axis of the elliptically 

 polarised light. Such an alteration was observed in the case of two 

 different crystals which were made the subject of experiment. 



The light reflected is shown to be exceedingly nearly plane 

 polarised, so that the absolute amount of change in the ratio of the 

 axes is small ; but the relative change is considerable, for tan w is 

 changed from 0*0334 to 0"0252. The change in the azimuth of the 

 major axis is not very large. As regards disturbing causes, it is 

 proved that moderate changes of temperature do not cause any very 

 perceptible alteration in the surface state. The experiments prove a 

 result unnoticed by Seebeck, that an emery-rouge polished surface 

 gives perfectly concordant results on repolishing, and in this respect 

 is quite as satisfactory as the chalk-polished surface that Seebeck 

 recommends. And in general the results of the paper tend to confirm 

 the views of Seebeck rather than those of Sir J. Conroy, for Seebeck 

 in his paper prefers polished surfaces because of the liability of the 

 natural surface to tarnish. 



