United States or French Southern Territories ? Vote for the TOP Country of the Week !
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.
This is by the beautiful method of rock-crystal prisms, not the Rochon method of double-image, but by thin wedges cut to given angles. I have told Mr. Alvan Clark my "experiences." and I hope he will apply his excellent mind to the scheme. I am insisting upon this point in some astronomical twaddle which I am now printing, and of which I shall soon have to request your acceptance of a copy.
Word Of The Day