I 



from dry clear air, betwceti the observer and objects a mile distant, 

 is sensibly polarized. 



With respect to the planes of polarization of skylight, he considers 

 that thoy may bo represented by a fiction of this kind : That there is 

 a certain amount of polarization due to the regular reflection of sun- 

 light from the sky in meridional planes passing through the sun and 

 the obsei-ver ; the polarization being most intense towards azimuth 

 90", and vanishing atO° and 180°. Combined with this polarization 

 is another, distinct from it, and represented by a more intense effect 

 due to reflection parallel to the plane of the horizon in all azimuths, 

 which unites with, modifies, and even overpowers the regular polari- 

 zation in meridional planes just referred to. 



The result will be the composition of the effects of the reflection 

 of light at a concave spherical surface having the sun for one pole, 

 with that due to reflection at a cylindrical surface perpendicular to 

 the horizon. 



If the latter be tolerably uniform in all azimuths, it will evidently 

 overpower and replace the former in points nearly opposite to the 

 sun, and which become visible when the sun is low. 



The author stated his conception of the physical cause for such an 

 arrangement of the planes of polarization to be, that whilst, at con- 

 siderable elevations, the number of reflecting particles is not so great 

 as near the horizon, the effect due to a single reflection will be the 

 less intense ; and, consequently, the horizontal reflection is generally 

 stronger than that in any other plane. But further than this, many 

 familiar facts shew that the horizontal vapours (or opaque particles, 

 of whatever kind they be, which occur in air), are, like a sheet of 

 paper, capable of receiving light and of emitting it, not necessarily in 

 the plane of reflection ; and such light, after several reflections nearly 

 parallel to the horizon and completely encircling it, — as we often see 

 it do when a slight whitish haze is visible all round, — reaches the 

 eye, much enfeebled, no doubt, by numerous reflections, but more in- 

 tensely polarized on that account, and reinforced by the number of 

 the reflecting particles. The lights thus irregularly reflected or 

 emitted by the horizontal strata of air, and again regularly reflected 

 by other particles in the same strata, will come to the eye more or 

 less polarized in planes parallel to the horizon. A similar action will 

 produce M. Babinet's second neutral point towards sunset ; and Mr 

 Forbes has remarked generally, after sunset, that the planes of pola- 

 rization no longer converge accurately to the luminary, but are nioro 

 or less twisted into a forced parallelism to the horizon. 



