390 Sir David Brewster on the Compensations of Polarized Light. 



of reflexion, while the whole of the refracted 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 em- 

 ployment of the light which they reflect may, in delicate experiments, be a 

 serious source of error, if we are not aware of its properties. By the principle 

 of compensation, however, we may convert this partially polarized light into 

 common light, and thus make experiments with as great accuracy in the day- 

 time, as we can do with the direct light of a flame. If the light from a parti- 

 cular part of the sky is admitted into a dark room, or otherwise employed, we 

 have only to compensate its polarization either by reflexion or refraction, and em- 

 ploy, as unpolarized or common light, that part of the light which corresponds 

 with the neutral line. 



3. The laws of the compensation of polarized light enable us to investigate 

 the polarizing structure of the atmosphere, and to ascertain the nature and ex- 

 tent of the two opposite polarizing influences, which I have found to exist in it, 

 and by the compensation of which the neutral points are produced. But, as I 

 shall soon submit to the Society the results of my observations on this subject, 

 I shall not add any thing further at present. 



4. In every case where reflected or refracted light reaches the eye of the 

 observer, whether it comes from bodies near us, or from the primary or secondary 

 planets of our system, the doctrine of compensation enables us to obtain im- 

 portant information respecting the phenomena presented by light thus polarized. 

 The nature of the reflecting or refracting surface, the angles of reflexion or 

 refraction, and the nature of the source of illumination, may, in certain cases, 

 be approximately ascertained. 



5. When the light of the sun, or any self-luminous body, is reflected from 

 the surface of standing water, such as the sea or a lake, it is polarized according 

 to laws which are well known ; but when the partially polarized light of the sky 

 (light polarizes in every possible plane, passing through the sun and the observer) 

 is reflected, a variety of curious compensations take place, which, when the 

 position of the observer is fixed, vary with the season of the year, and the hour 

 of the day. In some cases, there is a perfect compensation, the partially polar- 

 ized light of the sky being restored to common light by the reflection of the 



