of Polarized Light, and on a Polarimeter. 159 



in one plane was compensated by the polarization of the same light 

 in the opposite plane ; and, consequently, both the pencils, which 

 had undergone the two successive polarizing actions, had received 

 the same degree of polarization in opposite planes ; and in virtue of 

 these two equal and opposite polarizations, the light at the point of 

 compensation, where the system of rings disappeared, had been re- 

 stored from partially polarized to common light ; and the light on 

 each side of this point of compensation was in opposite states of 

 partial polarization. 



In order to give a distinct view of the nature of this experiment, 

 the author details the phenomena observed at particular angles of 

 incidence on glass. From the results at an angle of incidence of 

 24°, the ray suffering one refraction at 80°, and a second reflexion 

 at S3 5 , he concludes that the compensation which takes place is 

 produced neither by an equality of oppositely polarized rays, nor by 

 a proportional admixture of common light, but by equal and oppo- 

 site physical states of the whole pencil, whether reflected or re- 

 fracted. 



The remarkable phenomena produced at an angle of incidence on 

 glass of 82° 44' (at which angle cos (i + i') = cos 9 (i -f- i')), led the 

 author to the construction of what he terms the compensating rhomb, 

 consisting of a well-annealed rhomb of glass, or any other uncry- 

 stallized substance, having the angles of its base 130° 25' and 46° 35' 

 respectively, when the index of refraction is 1*525. When a ray of 

 light is incident upon the first surface at an angle of 82° 44', ex- 

 actly one-half of it is reflected ; and the other half, after refraction, 

 is reflected at the second surface, and emerges perpendicularly to 

 the adjacent surface, without suffering refraction; each portion 

 having, in the first instance, the same quantity of polarized light. 

 The second portion is found, on examination, to be in the state of 

 common light, although the ray at the second incidence consisted of 

 more than one-half of polarized light. Hence if the pencil, pre- 

 viously to reflexion at the second surface, consist of 145 rays of 

 polarized light, and 188 of common light, the effect of a single re- 

 flexion must be to depolarize polarized light, and to produce no 

 change whatever upon common light ; a property of a reflecting 

 surface never yet recognized, and incompatible with all our present 

 knowledge on the subject of the polarization of light. 



The author then describes an instrument which he has invented 

 for the purpose of accurately measuring the degrees of polarization, 

 and which he therefore terms a Polarimeter. It consists of two 

 parts ; one of which is intended to produce a ray of compensation 

 having a physical character susceptible of numerical expression, and 

 the other to produce polarized bands, or rectilineal isochromatic 

 lines, the extinction of which indicates that the compensation is 

 effected. The construction and mode of operation of this instru- 

 ment are, by the aid of figures, described and explained. 



The following is the general law established by these researches ; 

 namely, that the compensations of polarized light are produced by 

 equal and opposite rotations of the planes of polarization. Thus, 



