It is
based on the fact discovered by Sir David Brewster, that the light of
the sky is polarised in a plane at an angle of ninety degrees from the
position of the sun. It follows that by discovering that plane of
polarisation, and measuring its azimuth with respect to the north, the
position of the sun, although beneath the horizon, could be determined,
and the apparent solar time obtained. The clock consisted of a spy-
glass, having a nichol or double-image prism for an eye-piece, and a
thin plate of selenite for an object-glass. When the tube was directed
to the North Pole--that is, parallel to the earth's axis--and the prism
of the eye-piece turned until no colour was seen, the angle of turning,
as shown by an index moving with the prism over a graduated limb, gave
the hour of day. The device is of little service in a country where
watches are reliable; but it formed part of the equipment of the North
Polar expedition commanded by Captain Nares. Wheatstone's remarkable
ingenuity was displayed in the invention of cyphers which have never
been unravelled, and interpreting cypher manuscripts in the British
Museum which had defied the experts. He devised a cryptograph or
machine for turning a message into cypher which could only be
interpreted by putting the cypher into a corresponding machine adjusted
to reproduce it.
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