The instrument
is shown in fig. 1 (p. 280), and its principle is illustrated by fig. 2.
The scale is about 2/3.
E is the eye of the signaller; M the mirror; and L, S, fig. 2, a tube
containing at one end, L, a lens, and at the other, S, a screen of white
porcelain or unpolished ivory, placed at the exact solar focus of L: a
shade, K, with two holes in it, is placed before L. Let R, r, be portions
of a large pencil of parallel rays, proceeding from any one point on the
sun's surface, and reflected from the mirror, as R' r' (fig. 2). R'
impinges upon the lens, L, through one of the holes in K, and R' goes
free toward some distant point, O. Those that impinge on the lens will be
brought to a focus on S, where a bright speck of light might be seen.
This speck radiates light in all direction; some of the rays, proceeding
from it, impinge on the lens at the other hole in the shade K, as shown
in fig. 2, and are reduced by its agency to parallelism with r' and R',
that is, with the rays that originally left the mirror: consequently E,
looking partly at the edge of the lens, and partly into space, sees a
bright speck of light in the former, coincident with the point O in the
latter.
[Fig 1 and Fig 2, as described in the text].
What is true for one point in the sun's disc, is true for every point in
it. Accordingly, the signaller sees an image of the sun, and not a mere
speck of light, in the lens; and the part of the landscape which that
image appears to overlay, is precisely that part of it over which the
flash from his mirror extends; or, in other words, it is that from any
point of which a distant spectator may see some part or other of the
sun's disc reflected in the mirror.
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