Q1& 
Dr, Brewster on the Affections of Light 
ash, mother of pearl, bo?ies of a cod, quill, the human 7 iail, horn, 
tortoise shell, cornea of a fish, cornea of a cow, cornea of a maji, 
spermaceti, Rupert’s drops, gum Arabic, and caoutchouc. 
1. Rock crystal. The only specimen of this mineral which 
I could obtain when I made the preceding experiments, w^as in 
the form of a double convex lens about -j-oho 
thick. It exhibited only segments of the coloured rings, but 
they were very large and brilliant, and afforded me the means 
of making a very interesting experiment with a plate of agate. 
If a beam of common light is incident upon the neutral axis 
of this crystal, at such an angle that after reflection from its 
posterior surface, it shall emerge in the direction of its oblique 
depolarising axis, the light thus polarised by reflection and de- 
polarised by transmission through the depolarising axis will 
reach the eye in the state of white light. If this light is viewed 
through agate, one of the coloured segments, suppose green, 
will be distinctly visible; but if the agate is turned 90® round, 
the green colour will be converted into red, and in general the 
colour seen in one position of the agate will be complemen- 
tary to that which is seen in the other position. When the 
light, however, is brilliant, another very singular phenomenon 
presents itself. If the bright image seen through the agate is 
green, the nebulous image, in wdiich it is inclosed, will be red; 
and w'hen the bright image is red, the nebulous image will be 
green, and in general the colour of the nebulous image will 
be always complementary to that of the bright image. If we 
employ a prism of Iceland spar to examine the depolarised 
light, the colour of the ordinary image is always complemen- 
tary to that of the extraordinary image. We may therefore 
consider the preceding result as an experimentum crucis, which 
