principles of binocular vision. As we are
now, however, treating specifically of the
stereoscope and not incidentally, we shall go
into a little more detail, as to the history of
the instrument.

Although Professor Wheatstone's discovery
was alluded to in Herbert Mayo's Outlines
of Physiology in the year eighteen
hundred and thirty-three, it was not until
the twenty-first of June eighteen hundred
and thirty-eight that Professor Wheatstone
detailed the true theory of binocular
vision, together with a description and
diagram of his illustrative apparatus, which he
there first called the Stereoscope, (after two
Greek words meaning "solids—I see") before
the Royal Society, in a paper; for which, in
eighteen hundred and forty, he was awarded
the Royal Medal. The stereoscope was
afterwards produced and explained by Mr.
Wheatstone at the Newcastle meeting of the
British Association in September, eighteen
hundred and thirty-eight. The form of
instrument then exhibited remains to this day
the most efficient that has been constructed.
It is the most beautiful, because it is the
simplest; it is the most useful, because it can
be applied to the inspection of all drawings
made upon the stereoscopic principle,
whatever may be their size, and it is capable of
every kind of adjustment. A very little
exercise of ingenuity has sufficed to make it also
not less portable than any other, for it is made
on the lazy-tongs principle, and can be opened
and packed like scissors. Of this instrument,
when first shown to the British Association,
one literary journalist, expressing the opinion
of the time, now perfectly confirmed, said that
it rendered the phenomena of double vision,
about which volume upon volume have been
written, clear to the comprehension of childhood;
and by a contrivance so simple, that,
when once seen, any person can construct a
copy in an hour. The importance of the
discovery was recognised at once on all
sides.

In a report of that meeting of the
Association, published in the same year, it is
recorded, that "Sir David Brewster was
afraid that the members could scarcely judge,
from the very brief and modest account
given of this principle, and the instrument
devised for illustrating it, of its extreme
beauty and generality. He considered it one
of the most valuable optical papers which
had been presented to the section." Sir
John Herschel, on the same occasion, justly
characterised the discovery as "one of the
most curious and beautiful for its simplicity
in the entire range of experimental
optics."

At that time photography was an unheard-
of science, and there could be used in the
stereoscope only drawings made by the hand
of an artist. Geometric figures, and a few
simple sketches, could be made; but the eye
of the best artist was not accurate enough
to catch the delicate distinctions of outline,
light and shade existing in the same landscape
or figure, as it would appear seen from
two points at a distance of only two and a half
inches from each other. At the beginning of the
year eighteen hundred and thirty-nine,
photography became known, and Mr. Wheatstone,
not slow to perceive that the sun would
supply his stereoscope with pictures of the
necessary accuracy, soon obtained from Mr.
Talbot stereoscopic Talbotypes of statues,
buildings, and even living persons. The first
Daguerreotypes were produced for Mr
Wheatstone by M. Fizeau and M. Claudet.
The application of the stereoscope to photo
graphy having been communicated by Mr
Wheatstone to M. Quetelet, specimens being
at the same time sent, was made public in the
bulletins of the Brussels Academy for October,
eighteen hundred and forty-one. Eight or
nine years afterwards, Sir David Brewster
helped to popularise the idea by prompting
M. Dubosq Soleil (as we have elsewhere said) to
the construction of a number of stereoscopes
in which, by the use of a couple of semi-lenses,
with their edges directed towards each other,
a form of instrument was obtained very con-
venient for the Daguerreotypist, who deals
rarely in large pictures. This instrument is
a slight modification of the second form of
stereoscope—the refracting—suggested by
the original discoverer. The old reflecting
instrument, the first form, remains, however,
for all purposes of experiment and study, as
well as for many purposes of common use, by
far the best.

Before we proceed to an account of the
steps which led up to the discovery of the
stereoscope, and of some facts in nature
which it proves and illustrates, we should
say two or three words about the method
of investigation also illustrated by it. Mr.
Wheatstone is Professor of Experimental
Philosophy in King's College, London, and one
of the most successful of the experimental
philosophers of our own time. Down in the
vaults of King's College we remember seeing,
years ago, a great array of wires which we
were told belonged to an experiment of Mr.
Wheatstone's then in hand. Those wires
were the unborn electric telegraph, which
came into life out of the experiments of Mr.
Wheatstone on electrical velocity. The
discovery of the stereoscope furnishes an
interesting illustration of the method by which
the chief operations of experimental philosophy
are conducted. The surest way to get
a secret out of nature—if one is clever enough
to do it—is to overreach her: to entrap her
into a confession by compelling her to work
under unheard of conditions. She cannot
go to work on fresh material of your own
choosing without betraying some part of her
mode of setting about business. If all the
information that you want is not to be had
by playing the mysterious mother one trick,
try her with another and another. The