There is no difficulty in signalling
when the sun is far behind the back, if the eye-tubes are made to pull
out to a total length of five inches, otherwise the shadow of the head
interferes. For want of space, the drawing represents the tubes as only
partly drawn out. The instrument is perfectly easy to manage, and letters
can be signalled by flashes. Its power is perfectly marvellous. On a day
so hazy that colours on the largest scale--such as green fields and white
houses--are barely distinguishable at seven miles' distance, a
looking-glass no larger than the finger-nail transmits its signals
clearly visible to the naked eye.
I have made a makeshift arrangement on the principle of my heliostat,
using the object glass of an opera-glass for the lens, and an ordinary
looking-glass: the great size and short focus of the object glass is a
great convenience when using a mirror with a wide frame.
Professor W. H. Miller, the Foreign Secretary of the Royal Society, has
since invented a yet more compact method of directing the flash, which he
has described in the Proceedings of the Royal Society for 1865. It
consists of a plate of silvered glass, one of whose rectangular corners
is accurately ground and polished. On looking into the corner when the
glass is properly held an image of the sun is seen, which overlays the
actual flash. Beautifully simple as this instrument is, I do not like it
so much as my own, for the very fact of its requiring no "setting" is its
drawback.
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