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CAMPBELL'S REGISTERING SUN-DIAL.
Fig, 3. — May 5. 187Q. Two hours exposure, 6-inch lens, 30®. Passing clouds. The slope being out of po
the burning point got over the edge, and burned under the printing surface at the left sije in the figui
That night and next day the clouds con-
densed into heavy rain.
The blocks ought to be cut out in arcs of
circles to show the whole work of a day.
These are enough to show that the day's work
of registering sunshine may be printed after
sunset ; and published next day in a news-
paper, which was the object of this experi-
ment. A book set with the side towards the
cone is a plane set at a tangent to a sphere,
and cuts the cone at varying angles. Accor-
dingly the cone cuts through a varying num-
ber of pages. A block of wood set in the
same position is pierced to varying depths,
as shown in section on Figs, i, 2, 3. As the
sun moves, the section of the cone at the
printing surface changes till the last point
that is hot enough to mark goes off in a
hair line. The depth is not shown in printing.
This is the sort of work wbich a bottle or a
paper weight makes upon a table.
Sheets of gutta-percha placed in the same
positions are moulded by the conical point
of the pencil of rays, but the shapes will not
print. So are sheets of wax laid upon paper.
By these and by other such expedients it is
easy to get engravings, casts, and pictures of
sections of a cone of light, so as to estimate
the force of sunshine by shapes in substances
marked by the waves, as waves mark a
beach.
The cone of light formed by a sphere is a
very complicated structure, which is not easy
to understand or to explain. The central
point on the surface of the ball opposite to
the sun lets through a straight line of light,
which goes straight on till it is stopped. It
is like a ray of light shining through a pin-
hole. Take a school globe for illustration,
call that point the pole, and the line the
axis. A ring of the glass surface close to the
point bends the waves of light into a cone
which crosses the axis at a distant point.
Larger outer rings, comparable to parallels of
latitude, bend " rays " or waves to points upon
the axis nearer and nearer to the glass, and
the outermost rings, which refract, or bend
rays or waves, bring them together close to
the glass. A whole series of circular discs,
images of the sun, or " foci," are strung upon
the central line like buttons on a string.
All these images which together answer to
the lead in a pointed drawing pencil are hot,
and the hottest are those which are formed
by the largest rings, which are the farthest
from the pole. The ring between latitude
45° and 50° is far larger than between 85°
and 90°. In a cone an inch and a half long,
and about an inch wide at the glass which
has a diameter of nearly six inches, that part
which answers to the lead in a pencil, and is
hot enough to burn blackened cardboard,
was found experimentally to be nearly an
inch long about noon, on a clear London day
at the end of April. On the 29th a hot
point pierced twenty-two sheets of a note-
book. It scorched the first sheet to a width
of six-tenths of an inch. On the 17th the
cone pierced fourteen sheets and scorched
the fifteenth. That is a measure " of longi-
tudinal spherical aberration," which is ex-
plained scientifically in works on Optics, and
was proved experimentally in April, 1879.
But there is a further complication in the
cone. Waves of light of different measured
lengths, which are detailed in scientific works,
give different colours, and meet upon the
axis at distances proportioned to their " re-
frangibility." The red rays, or the rays
which seem red to human eyes, cross farther
from the glass than the rest of the visible rays.
But invisible hot rays cross still farther away.
That " chromatic aberration " belongs to each
focus in the series of images which result
from " spherical aberration," that is from the
shape of a glass which is a sphere, or part of
a sphere. I have found that within a coni-
cal figure, an inch and a half long and an
inch wide at the base, heat varies from 110°
to 700° at least. By wearing black spectacles
a good deal of this may be seen upon paper
screens in safety. By casting shadows with
pins stuck into paper set to cut the cone
lengthwise, the direction taken by the light
in the cone, before and after the waves have

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