2q 6 Dr. Brewster on the Polarisation of Light. 
tion CAB, so as to increase the obliquity of the light radiating 
from O, the corner A will conceal the bright image at O, 
while the faint secondary images continue visible. . If we now 
examine these images with a prism of Iceland spar, they Will 
be found to be polarised exactly like the bright image ; but 
upon increasing the obliquity of the incident rays still more, a 
nebulous light makes its appearance in the place of the faint 
images, and what is very singular, this nebulous light is polarised 
in the opposite manner from the bright image^ and the bright image 
has ihe_ same relation to this nebulous light, as the bright image 
has to the iiebidous image of the agate, or as the first pencil has to 
the second pencil formed by doubly refracting crystals. 
When a beam of light from a candle is transmitted through 
a convex lens, and falls upon a series of plates at- a consi- 
derable angle of incidence, a bright image A, and a row 
of faint images cd are distinctly visible, as in fig. 2. If we 
i 
now view these images through a prism of Iceland spar, they 
will all be doubled; but upon turning round the prism, the - 
image A will vanish, and along with it the faint images ef, and 
after another quarter of a revolution the im.age B will vanish,' 
and along, with it the faint images cd, the images A, and ef - 
having reappeared. When the angle of incidence, however, 
is greatly increased, the secondary images cd always vanish 
along with their principal image A, and ^ along with their 
principal image B.‘ 
The polarisation of the nebulous light, in an opposite man- 
ner from the bright image, first shewed itself to me in a very 
curious phenomenon. When the nebulous light and the bright ’ 
image were both visible at the same time, and were trans^ 
mitted through topaz, so as to form the elliptical coloured ‘ 
