280 



an instrument termed the Compensating Rhomb, by means of 

 which he considers decisive evidence of the correctness of 

 his views has been obtained. 



In order to determine if this principle be general, and to 

 ascertain the laws of the compensation of polarized light, 

 Sir David Brewster constructed an instrument for measuring 

 the degrees of polarization. This he calls 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 rectilinear isochromatic lines, the extinction of 

 which indicates that the compensation is effected. The de- 

 tails of the construction of the instrument are fully given in 

 the memoir, and numerous experiments made with it, and 

 confirmatory of the author's views, are described. 



In conclusion, Sir D. Brewster points out as the general 

 results of his inquiries, as follows: 



*' 1. The first and most important result of this inquiry 

 is, that it affords a new and independent demonstration of the 

 laws of the polarization of light by reflexion and refraction, 

 given in my papers of 1830. As this result has been already 

 referred to, I shall merely mention the following general 

 proposition. 



" When a ray of common light is incident at any angle 

 upon the polished surface of a transparent body, the whole 

 of the reflected pencil suffers a physical change, bringing it 

 more or less into a state of complete polarization ; in -virtue 

 of which change, its planes of polarization are more or less 

 turned into the plane of reflexion, while the whole of the re- 

 fracted pencil has suffered a similar, but opposite change, in 

 virtue of which, its planes of polarization are turned more or 

 less into a plane perpendicular to the plane of reflexion. 



" 2. As the light of the sky and the clouds is more or less 

 polarized, the employment of the light which they reflect 

 may, in delicate experiments, be a serious source of error, if 



