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we are not aware of its properties. By the principle of com- 

 pensation, however, we may convert this partially polarized 

 light into common hght, 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 particular part of the 

 sky be admitted into a dark room, or otherwise employed, we 

 have only to compensate its polarization either by reflexion 

 or refraction, and employ, 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 extent of the two opposite 

 polarizing influences, which I have found to exist in it, and 

 by the compensation of which the neutral points are pro- 

 duced. But, as I shall soon submit to the Society the results 

 of my observations on this subject, I shall not add any thing 

 furthjer 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 important 

 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 ap- 

 proximately 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 polarized in every possible plane, passing through the 

 sun and the observer) is reflected, a variety of curious com- 

 pensations take place, which, when the position of the ob- 

 server is fixed, vary with the season of the year, and the hour 



2 A 2 



