ings and rockings of the vessel have caused
the mast head to describe a circle or an oval,
as the case might be. In the language of the
same hypothesis, polarised light is light
propagated only by one plane of vibrations;
the effect of whatever causes polarisation,
being, to suppress the vibrations in the plane
at right angles to the former. Hence, they
say, the different properties possessed by the
opposite sides or poles of the ray. The theory
is beautifully ingenious; but, if the existence
of the other be more than doubtful, soon to
be classed with the fixity of the earth and the
crystal orbs of the older astronomers, what
becomes of all these complicated vibrations?
Light polarised by reflection is rarely applied
to microscopic purposes.

Secondly, Light may be polarised by
transmission through a bundle consisting of
from sixteen to eighteen plates of thin
glass. Of this nature is the polariscope
employed in Woodward's hydro-oxygen
microscope.

Thirdly, Light is polarised by passing
through certain transparent crystals. Some
of these, called double-refracting crystals,
split the ray in two. Place them over an
object—a printed paragraph for instance—
and you suddenly see double; duplicate
paragraphs astonish your gaze. They are
carried to your retina by the divided ray, and
each half-ray is polarised. Iceland spar is
the crystal generally employed by the
microscope maker for the prisms already
mentioned, although others would serve. By an
ingenious optical operation, only one of the
half-rays is allowed to traverse the body of
the microscope. By interposing between the
two prisms a plate of selenite or other
doubly-refracting medium, colour is produced
by "interference," in undulatory language,
by turning the moveable collar of the
polariser, the polarised ray is made to revolve,
and an extraordinary succession and variety
of hues is the result. These effects will be
produced, as far as the ground tint is
concerned, even if the objects through which
the light is transmitted to the eye have
themselves no polarising influence; but, if
they have, other phantasmagoric effects
will be developed, of which no conception
can be conveyed by printed words. The
eye actually cloys of the spectacle, if
long-continued; dazzled and spent with an
alternating contest of iridescent hues, it is
glad to repose on the homely colouring of
things as they appear in their rainy-day
dress.

    "Where'er I peep, whatever sights I see,
    My heart, untravelled, still returns to thee;
    Still to fair daylight turns, quintessence pure."

Amongst my private treasures is the
compound eye of a beetle, parts of which change
colour under polarised light. It would be
curious to ascertain whether any individual
creatures—including certain of mankind—are
not gifted with eyes that are more or less
polariscopes.

If there exist insects or crustaceans,
whose eyes, besides being microscopes, are
also polariscopes, what a highly-seasoned
view of nature they must have, compared
with ours! We hear of cases of people being
affected by colour-blindness, as if the grey
ray were the only one that reached their
retina,—of mercers confounding green with
scarlet, and of shopmen obliged to have their
coloured skeins of silk ready sorted to their
hand over-night. We have the phenomenon
of painters whose pictures make perfect
engravings,—they are irreproachable in respect
to light and shade, perspective, and drawing,
—but in point of colour, look like the work
of madmen. We have aged oaks rearing
their azure stag-heads into a cloudless grass-green
sky, and overshadowing a group of
yellow bandits who fiercely bestride their
purple steeds. Most of our integuments exert
a marked action on polarised light; one
would think that, in the case of those artists,
the capricious faculty was extended to the
integument of the eye.

As to what special objects polarised light is
applicable—like the microscope itself—it
embraces every material thing in nature,
whether belonging to the animal, the
vegetable, or the mineral kingdom. It is
recommended to examine everything with
polarised light, in the certainty of its leading to
valuable discoveries; by it, the internal
structure of various transparent objects is
rendered evident, although they may not be
recognisable by ordinary illumination; by its
delicate indications, the science of optics has
become the handmaid to almost every other
branch of physics. Integumentary substances
in particular form a brilliant and interesting
class of objects. A section of a horse's hoof
has the effect of the richest Brussel's carpet,
with a symmetrical pattern that might be
copied by the loom; the same of the
rhinoceros's horn, which, however, is said not to be
horn, but a tuft of hairs naturally glued together.
Ram's horn, a deer's hoof, sheep's hoof,
have each its characteristic elegances. If the
substance, called whalebone, could be made to
display, when beamed on by the rays of gas
or wax candles, the ornamental structure and
the harmonious shades which it offers when
viewed by the micro-polariscope, it would
soon become the fashion for ladies to wear——
dare I write it?—stays outside, instead of
beneath, their dress.

The elegant structure of fishscales is
admirably seen by means of the polariser.
Agassiz has classed fish according to their scales;
and the student should have a representative
of each class for comparison. Perhaps the
most striking are the ctenoid, or comblike,
scales; namely those which have rows of
teeth at the edge by which they are
attached to the skin, as in the sole, the
pike, the perch, and the red mullet. The