124 REPORT—1840. 
tion of this phenomenon*, that it occurs when the air is ex- 
tremely clear and highly charged with humidity (as we should 
expect from (254.)), and that the rays which then reach the 
mountain have undergone total internal reflexion in the higher 
and moister strata of the atmosphere. 
258. derial Shadows.—We do not by this refer to shadows 
of persons thrown on clouds surrounded by coloured glories, of 
which we will afterwards speak, but of shadows of clouds and 
other objects projected to a great distance in the air, and which 
being rendered visible by its imperfect transparency, produce cer- 
tain remarkable effects of perspective. The diverging rays so 
often seen proceeding from the sun, when near setting, are of this 
kind; and the corresponding fact of rays (or clear intervals 
between the shadows of clouds), which appear to converge to a 
point diametrically opposite to the sun. This rarer phzeno- 
menon we have twice seen; once, combined with a rainbow, to 
whose centre of course the rays were directed ; and lately, from 
the summit of Goatfell in Arran, whence the rays appeared 
directed to a point in the sea, and converging from all sides of 
the circumference. We chiefly mention the circumstance to 
call attention to a curious and elaborate paper by Professor 
Necker+, of Geneva, who undertakes to prove that diverging 
solar rays are sometimes produced by very distant mountains, 
and that they thus picture forth, to inhabitants of our country, 
spectral outlines of mountain-chains in another, far removed 
from direct vision. 
259. Polarization of Sky-light.—Sir D. Brewster appears 
to have been the first to remark that the light of the blue sky 
exhibits traces of polarizationt. I apprehend that it must be 
difficult to assert that the blue rays are actually so polarized, 
for the polarization of the white light, with which no doubt the 
blue is diluted, would produce the effects observed. I do not, 
however, doubt that such is the case. 
260. M. Arago determined the polarizing angle for air to be 
45° nearly §, and the maximum polarization of the sky to be 
90° from the sun’s disc, the polarization being in a plane passing 
through the eye and the sun. It is a singular fact, and one 
difficult of explanation, that, proceeding to a greater distance 
than 90° (in a vertical plane), the polarization diminishes, be- 
comes zero, and reappears in a plane perpendicular to the 
* British Association, Seventh Report, Sections, p. 10. See too a paper by 
Prof. Necker, of Geneva, Phil. Mag., 3rd series, i. 335. 
+ Annales de Chimie, Fev. Mars, 1839, and Bibliotheque Universelle, 
xxiil. 355. 
t Treatise on New Philosophical Instruments, p. 350. Edinburgh, 1813. 
§ Biot, Traitée de Physique, iv. 289. 
