tom of the microscope, and the body-prism
screws inside, the object-glass screwing
beneath it and outside. The selenite is laid on
the stage, and on it the object; the focus is
found; and you have then only to peep your
fill, causing the polariser to revolve occasionally.
In many French microscopes, and in
certain English ones, the analyser, whether a
prism or a tourmaline, is fitted to the
eyepiece instead of to the lower end of the body
of the microscope; but in either case it is
still above the object-glass. These details are
not amusing, but they will be welcome to
unpractised manipulators, who are puzzling
over a newly-arrived instrument, which
their love of natural history has induced
them to order.

But I may be expected to answer the
reasonable question, "Pray, what is polarised
light?" The reply is ready; "I don't
exactly know; nor do I know who does
know exactly." The term polarised, as will
be explained by-and-by, affords no explanation,
description, or clue. Polarised light is
light that has been subjected to certain
modes of treatment, by which it acquires, or
more properly loses, certain qualities. This
is not a very precise or graphic definition,
but I cannot help it. There are secrets of
nature which lie beyond mortal ken.
Polarised light is a sort of superfine light,—to
use familiar terms,—from which all the
coarser properties have been winnowed,
strained, sifted, or beaten out. If common
light were wheat immediately after being
ground between the millstones, polarised
light would be the finest flour obtained therefrom.
Light, after having undergone a
certain amount of discipline, or torturing, is
said to be polarised; about which process of
polarisation great and doughty battles might
be fought. But, as no professor has plunged
as yet to the truth-containing bottom of the
well of light, I content myself with the
undeniable statement that polarised light is a
very pretty thing. Fancy yourself living in
a region solely illuminated by Auroræ
boreales—and it is not proved that polarisation
has nothing to do with the said Auroræ,—
imagine a country where every passing cloud
throws a diverse-coloured shadow of gorgeous
hues across your path; where the air breeds
rainbows without the aid of a shower, and
where the summer breeze breaks those
rainbows into irregular lengths, fragments, and
glittering dust, scattering them broad-cast over
the land, like autumnal leaves swept by a gale
from the forest, and you have an approximate,
and by no means exaggerated idea of
the effects of polarised light on substances
capable of being affected by it. For, it is
light endowed with extra delicacy, subtlety,
and versatility. It renders visible minute
details of structure in the most glaring
colours; it gauges crystalline films of
infinitesimal thinness; it betrays to the student's
search, otherwise inappreciable differences of
density or elasticity in the various parts of
tissues. Indeed, as a detector, polarised
light is invaluable, acting the part of a
traitorous spy under the most unexpected
circumstances. It denounces as cotton what
you believed to be silk; it demonstrates
disease where you supposed health. It
adorns objects that are vile and mean, whose
destiny is only to be cast out—such as
parings of nails, shavings of animals' hoofs,
cuticle rubbed or peeled from the stems of
plants, offscouring of our kitchens and storerooms,
sugar, acids, and salts—with the most
magnificent, the most resplendent tints,
such as are seen when the sun streams
through the stained glass windows of a Norman
cathedral.

Light is thrown into this magical condition.
First,—When it is reflected from glass at an
angle of incidence of fifty-six degrees, forty-five
minutes from the perpendicular. This
only describes one of the modes of producing
polarised light, and is no answer to the question,
"What is it?" It was thus that the phenomenon
was actually discovered in eighteen
hundred and eight, by professor Malus,
while viewing, through a doubly-refracting
prism, the light of the setting sun reflected
from the glass panes of a French window,
called a croisée, which happened to stand
open, like a door on its hinges, at an angle
which must have very closely approximated
to that which has since been ascertained to
be the polarising angle for glass. The
ray so reflected is found to have acquired the
property of possessing different sides. If the
original ray be supposed to be a cylindrical
rod, polished or white all round, which is
capable of being reflected from a polished
surface whatever part of its circumference
may strike that surface, the polarised ray
may be compared to a square-shaped rod with
four flat sides, two of which (opposite) bright
and polished, are capable of reflection, while
two—black or dull—are not. Now, the word
"poles," in physical science, is often used to
denote the ends or sides of any body which
have acquired contrary properties, as the
opposite ends of a magnet, which are called the
positive and negative poles. By analogy, the
ray of light, whose sides lying at the right
angles with each other, were found to be
reduced with opposite physical properties,
was said to be polarised. The term remains,
and can scarcely be changed now; but it
subsists in books as a monumental specimen
of unfortunate nomenclature. On the
undulatory theory, common light is assumed to
be produced by vibrations of the ethereal
particles in two planes at right angles to the
progress of the wave; there are perpendicular
vibrations, and there are horizontal vibrations—
which is analogous to the motions of
the waves of the sea, as experienced by those
who have crossed the Channel in a steamboat
during a brisk gale, when the rectangular
vibrations occasioned by the alternate pitch-