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A^Av i^-a ■ C^,
A T some time in March or April, iSyg,
•^*- some gentlemen, and some ladies too,
were very much amazed to find their writing-
desks and tables smoking. They wrote to
the papers about it. Under the heading,
" An unsuspected danger," a series of letters
appeared in the leading journals. Certain
transparent glass balls, now commonly used
as paper-weights, being set in sunshine,
" acted like a burning-glass " on tables and
table-covers, to the astonishment of their
owners. They are burning-glasses. In daily
life theory very seldom is practised ; know-
ledge very seldom is used. Though "optics"
are taught in schools and colleges, very few
people realise that things polished and
transparent, with surfaces shaped so as to
condense light, are instrumenls which may
set combustibles on fire.
" Burning glass " was defined in the " En-
cyclopedia Britannica " in 1797, as " A con-
vex glass, commonly spherical, which, being
exposed directly to the sun, collects all the
rays falling thereon into a very small space
called the focus, where wood or any other
combustible matter being put, will be set on
fire." In the article quoted Sir David
Brewster tells what had been done with
burning-glasses. We have all been taught
that Archimedes burned the Roman fleet at
Syracuse with sunshine, about 2,090 years
ago. The writer's family and Sir David
Brewster were friends, and he had the great
advantage of knowing that distinguished
philosopher, and of learning curious know-
ledge from him in conversation. Some fifty
years ago somebody gave a child an optical
toy, a glass like the bulb and stem of a big
thermometer filled with water. He then
learned experimentally that a transparent
ball is a magnifier, and burns fingers. Ever
since that childish lesson was learned, as boy
and man the writer has been striving to learn
more about " burning-glasses." Neverthe-
less he too was caught unawares by sunshine.
In April, 1879, an innocent egg-shaped
water-bottle burned a hole in a toilet-table,
which happened to be set in a new place
where the sun happened to shine upon it at
the hour when the focus of this burning-
glass happened to be at the table on which
the bottle stood. That particular combi-
nation may not happen again for a year, but
every time such a combitiation does occur
danger recurs, from the misuse of a bottle.
The associate of this criminal found guilty of
arson being suspected, a tumbler was tried
and convicted of the same offence. Dragged
into light he too burned wood. A knot in
a pane of glass is a " lens ;" a finger-glass, a
wine bottle, a tumbler, a wine glass, a globe
for gold fish, a chemist's window ornaments,
any glass or glass vessel full of clear fluid and
properly shaped, may happen to be so placed
as to concentrate sunshine at some hour of the
day, on some day in a year, upon a combustible, ,
when a breeze of wind may kindle a blaze,
and buril a house or a ship unexpectedly
and Unsxlspected. Knowing something of
this, about 1853 the writer tried to use some
of the small amount of knowledge which he
had picked up. Amongst other contrivances,
he then invented a very simple instrument
founded upon two facts, (i) a transparent
ball is a burning-glass; and (2) the world
turns round. This paper is an attempt to
describe the principle of the contrivance
which a child of six years old understood
some fifty years ago.
A billiard ball is a " sphere." Slices cutoff
it are bounded by circles. All sides of it are
alike. On it, or any other " sphere," great or
small, may be drawn the lines and scales which
are drawn upon school globes. A trans- fj} '
parent sphere placed where any sort of light 4 ~*
shines upotl it presents the same curves to «.^ /~^A —
the "rays," and bends them to correspond- ^^^^^
ing places on the opposite side, where the AmjI
lays cross at a " focus." Whether a sphere ^^^^
is turned end for end, or capsized, spun, or /) «.
rolled, the focus for parallel rays always is
opposite to the source of light, and at a
certain distance from the surface. A spherical
bottle is a lens ; and so is the atmosphere.
When a transparent sphere is set out of
doors all the shining bodies that stud the
visible sky shine through it to opposite foci.
Each forms an image of itself and a cone of
light, which may be cut by a surface placed
in the cone. The lens of a photographic
camera is founded upon the principle. The
sensitive screen cuts cones of lightj The
writer has got images of the starsj the moon,
and the sun, by photography. But the
sc«i^en of a camera is flat, not spherical.
Distances measured upon it are unequal, and
• drawings are out of perspective, and out of
proportion. The stars and the moon draw,
hilt the sun is so hot that the image " burns."
For many years a lens has been set at Paris
so as to focus the sun's rays upon gunpowder,
at noon, and so fire a small cannon. Gun-

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