200 Dr. Brewster on the Affections of Light 
crystals possess in two positions the singular faculty, of depo- 
larising light, or of depriving it of the property which it ac- 
quires by transmission through tlie agate, while in other two 
positions of the depolarising crystal, the polarity of the light 
suffers no change. Thus in Plate V., fig. 5, let ABDC, be a 
piece of mica or of any other crystallized body interposed 
between a plate of agate and a prism of Iceland spar when 
one of the images has vanished, and let GH be parallel or per- 
pendicular to the laminae of the agate when the vanished image 
continues invisible. This line I have called the neutral axis, as 
no effect is here produced upon the polarised light. By turning 
the mica round, the vanished image will gradually appear> 
and when the line AD comes into a vertical position, it will 
be restored to its full lustre, and will never again vanish what- 
ever be the position of the Iceland spar. The line AD I have 
therefore called the depolarising axis, as the light in passing 
through it has been deprived of the polarity communicated 
by the agate, and which prevented it from penetrating the 
rhomboid of Iceland spar. 
By continuing the motion of the mica, it will be found that 
EF is also a neutral axis, and BC a depolarising axis. The 
depolarising axes are common' to almost all crystallized sub- 
stances, and what is very singular, I have discovered them 
in horn, gum Arabic, glue, tortoise-shell, caoutchouc, gold 
beater's skin, amber, mother of pearl, camphor, spermaceti 
melted and cooled, bees' wax melted and cooled, adipocire 
melted and cooled, manna, oil of mace, acetate of lead melted 
and cooled, human hair, bristles of a sow, human cornea, 
cornea of a fish, cornea of a cow, and imperfectly in some 
pieces of plate glass. 
