Reflexion of Light from Iceland Spar. 



243 



us with two independent tests of a change of surface, and completely 

 determine the nature of the light, so that a knowledge of their values 

 before and after polishing enables us to state the precise alteration 

 produced in the reflected light, a question which has never been 

 investigated, and is, I believe new to this paper. 



II. Apparatus. 



A series of preliminary experiments were made to discover what 

 apparatus was best suited for the investigation. I found that, 

 whether a Nicol or a Nicol and a quarter undulation plate were 

 used as analyser, it was best to polarise the light before incidence. 

 Also, observations showed me that a Nicol and a quarter wave plate 

 were a more sensitive arrangement than a simple Nicol, supposing 

 that in each case the light was polarised before incidence. I have 

 therefore deemed it necessary to employ the Nicol and quarter wave 

 plate arrangement in order to secure all the accuracy that is possible 

 by completely quenching the reflected light. 



The instrument I used was an elliptic analyser kindly lent for the 

 purpose of these experiments by Professor Stokes. 



A very full description of the instrument will be found in the 

 ■ Phil. Mag.' for 1851, but the following abbreviated account is given 

 in order to explain the way in which it was used during the course of 

 the experiments. 



The elliptic analyser consists of a brass annulus attached to a 

 vertical stem which fits into a hollow cylindrical foot. When the 

 foot is placed on a table, the plane of the annulus is vertical. Within 

 the annulus turns a brass graduated disk ; and the angle through 

 which it turns is read off by means of verniers engraved on the 

 annulus. These verniers are therefore fixed. The disk is pierced by 

 a central aperture on the side of which opposite the incident light is a 

 screw thread, so that a cell containing a quarter wave plate can be 

 screwed into the disk. In front the disk carries a hollow cylinder 

 turned in the lathe with the disk itself. Round the cylinder turns a 

 collar into which is screwed a tube containing the analysing NicoL 

 The collar carries a pair of level edged verniers by which the angle 

 may be read off through which the Nicol has been turned. These 

 verniers are therefore moveable. Thus the quarter wave plate moves 

 in azimuth, carrying the Nicol along with it, and the Nicol has 

 likewise an independent motion in azimuth. In observing, the light 

 is extinguished by a combination of the two movements, in which 

 case the elliptically polarised light is converted by the quarter wave 

 plate into plane polarised, which is then quenched by the Nicol. 

 There are two principal positions in which the light can be quenched, 

 and, since either Nicol or quarter plate may be reversed by turning 

 through 180°, there are four subordinate positions corresponding to 



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