had been no blue glass whatever. The
distinct properties of the yellow and blue
rays are manifested as strongly in the
germination of plants. Germination is prevented
by the action of the yellow ray, while to the
blue ray it is mainly indebted.

The rays that have passed through to form
the picture, have been called the photogenic
rays: they refract not quite in the same way
as the luminous or colorific rays, and therefore
the focus of the photogenic picture and
that of the picture thrown on the ground
glass will not exactly coincide. For this,
allowance has to be made in practice, and
accurate instruments for ascertaining the
true photogenic focus have been invented,
one by M. Claudet, and another by Mr.
G. Knight. They are called Focimeters.
There are hidden mysteries, however,
connected with this portion of the subject.
Means have been already here and there
discovered, by which the colours of the
spectrum may be printed at once on
photographic tablets, and the sun—most brilliant
of artists—may paint his pictures at the same
time that he is engraving them. The process
is not yet disclosed. Mr. A. Hill, of New
York, affirms that he has taken many pictures
from Nature, having all the beauty of natural
colouring upon them. A new material is
said to have been introduced in aid of this
effect. When all mechanical details have
been perfected, we may therefore expect this
new step to be made publicly, by which
Apollo will be raised above Apelles in the
world of art.

The application of photography to the
stereoscope produces an extremely pretty
toy, that is of no use except as an elegant
and valuable illustration of a train of scientific
reasoning. The instrument itself was
invented some years since by Professor
Wheatstone, to illustrate his discovery of
the principles of binocular vision. In 1849
Sir David Brewster exhibited to the British
Association at Birmingham a stereoscope
adapted to the inspection of daguerreotype
pictures. Afterwards he happened to
describe the instrument to an optician in Paris,
M. Duboscq Soleil, who being an enterprising
man, constructed a number of such
instruments on speculation. At the beginning of
1851 some of these were exhibited at one of
the soirées of Lord Rosse; they excited attention,
and the photographers of London,
seizing the notion, very soon began to take
stereoscopic portraits. In the stereoscope
two exactly similar pictures are placed side
by side under a pair of prisms, which are
so adjusted, that one image falls on each eye,
and the images on the two eyes do not fall
on precisely corresponding parts. This gives
the idea of distance.

For it is to the use of two eyes that we are
indebted for the facility with which we derive
ideas of form, solidity, and distance. There
is only one point before us, to which both
eyes can be turned in the same way at the
same time. Every other point before and
behind that will fall upon both eyes, will
fall upon the retina of each eye in a different
place, and the amount of variation presents
itself through the optic nerve to the brain as
the idea of distance. Upon this hint the
stereoscope is formed, and the effects of
roundness and distance are presented to the mind
by a pair of flat photographic pictures. M.
Claudet has constructed an ingenious variation
on the ordinary stereoscope, by placing under
it two plates not perfectly identical. In one,
for example, there are two men fighting: one
strikes, the other wards. The companion
plate contains precisely the same men; with
this difference in their attitude, that the one
who struck now wards, and the aggressor
stands on the defensive. In looking at this
group, and at the same time rapidly moving
to and fro a small slide behind the glasses,
which covers now one eye and now another,
the two impressions run into each other and
produce the appearance of an active sparring
match. Again, a needle-woman, represented
on one plate with her needle in her work,
and in the other with her thread drawn out
to its full length, appears, when the slide
is shifted to and fro, to be industriously
sewing.

Among ingenious contrivances we ought
not to omit to rank Mr. Mayall's very neat
method of producing what are called crayon
portraits in daguerreotype. His plan is
to place between the sitter and the camera
a revolving plate, having a hole cut into
the middle of it, from which there proceed
broad rays as of the sun upon a signboard.
The result is a picture upon which the head
is engraved with unusual distinctness, and
the bust is gradually shaded down into the
general colour of the plate, so that the
effect is that of a crayon portrait.

Photographic processes on glass and paper
are even more valuable as aids to knowledge
than daguerreotypes.

There are many processes by which pho-
tographic impressions may be taken upon
paper and glass; a book full of them lies
at this moment before us: we have ourselves
seen two, and shall confine ourselves to the
telling of a part of our experience. We rang
the artist's bell of Mr. Henneman in Regent-
street, who takes very good portraits upon
paper by a process cousin to the Talbotype.
By that gentleman we were introduced into
a neat little chamber lighted by gas, with
a few pans and chemicals upon a counter.
His process was excessively simple: he
would show it to us. He took a square of
glass, cleaned it very perfectly, then holding it
up by one corner with the left hand, he poured
over the centre of the glass some collodion,
which is, as most people know, gun-cotton,
dissolved in ether. By a few movements of
the left hand, which appear easy, but are
acquired with trouble, the collodion was