Editing Charles Wheatstone (section)

=== Polar clock ===
One of Wheatstone's most ingenious devices was the 'Polar clock,' exhibited at the meeting of the British Association in 1848. It is based on the fact discovered by Sir [[David Brewster]], that the light of the sky is [[Polarization (waves)|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 spyglass, having a [[Nicol prism|Nicol (double-image) prism]] for an eyepiece, and a thin plate of [[Selenite (mineral)|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 eyepiece 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 1875–1876 [[British Arctic Expedition|North Polar expedition]] commanded by [[George Nares|Captain Nares.]]