244 SIR DAVID BREWSTER ON THE 



In both these azimuths E is ^° with common light, and is yellowish red. 



Azimuth 0°. Light polarised 0° and 90°. E' is polarised 84° to the right. 

 0' vanishes with C, and is fainter and redder than E'. 



Azimuth 1 80°. Light polarised 0° and 90°. E' is polarised 90° to right. 

 vanishes with 0, and is fainter and redder than E'. 



B 1. Face inclined 5° 28' to B. 



In Azimuth 0°. With bright sun-light the spar pencil E is distinctly polarised 

 about 14° to the right. 



Azimuth 0°. Light +45 c — 45°. E' and 0' polarised about 13° to left. 



Azimuth 0°. Light polarised 0° and 90°. E is polarised about 50° to the right, 

 and 0' vanishes with 0. 



B 2. Face inclined 12° 8' to B. 



With common light, a small quantity is polarised in the plane of reflexion. 



As the azimuth approaches to 90° on either side of the principal section, the 

 light is polarised about 90° out of the plane of reflexion, much bright blue light 

 being left. At small incidences the blue becomes brighter and purer. The light 

 is orange when the principal section of the analysing rhomb is in the plane of 

 reflexion, as if red light was polarised 90' out of the plane of reflexion, and blue 

 light in that plane. 



The experiments described in the preceding pages form but a small portion 

 of those which I have made, both with artificial and solar light on the action of 

 the surfaces of calcareous spar on common and polarised light. In submitting 

 them to the Society, it is proper that I should mention the great difficulty of 

 obtaining precise results in such observations. The extreme faintness of the re- 

 flected light ; its imperfect polarisation in many cases at* the angle of maximum 

 polarisation ; and the loss of one-half of the light in the analysing prism, render 

 it very difficult to determine the deviation of the reflected pencil, especially 

 when it is partially polarised, or unequally double ; and I have been surprised at 

 the great difference in the results obtained at different times with the same 

 surfaces, when the observations were in both cases recorded as satisfactory. 



The following general results, however, are sufficient to show the importance 

 of this class of researches, in reference to certain questions in the undulatory 

 theory which have not yet been solved, and perhaps to guide the mathematician 

 to their solution. 



1. In the reflexion of light, the surfaces of calcareous spar in contact with 

 fluids, act in some cases as ordinary uncrystallised surfaces in the polarisation of 

 light. 



2. In reflecting common light, they polarise it out of the plane of reflexion, 



