New Eighty-one Tun Gun. portion. Tkat the 81 tun gun will ultimately have a caliber cal tube is so fitted that it freely moves round It own axis ; this broad end le closed by another glass disk in the center of which ie a small star or other figure formed of thin films of selenite exhibiting when examined with polarized light strongly contrasted colors ; and a hand is painted in such a position as to be a prolongation of one of the principal sec tions of the crystaline films. At the smaller end of the con ical tube a Nicol's prism is fixed so that either of its diago nals shall be 45° from the
New Eighty-one Tun Gun. portion. Tkat the 81 tun gun will ultimately have a caliber cal tube is so fitted that it freely moves round It own axis ; this broad end le closed by another glass disk in the center of which ie a small star or other figure formed of thin films of selenite exhibiting when examined with polarized light strongly contrasted colors ; and a hand is painted in such a position as to be a prolongation of one of the principal sec tions of the crystaline films. At the smaller end of the con ical tube a Nicol's prism is fixed so that either of its diago nals shall be 45° from the principal section of the selenite films. The instrument being so fixed that the axis of the conical tube shall coincide with the polar axis of the earth and the eye of the observer being placed to the Nicol's prim it will be remarked that the selenite star will in general be richly colored ; but as the tube is turned on its axis the colors will vary in intensity and in two positions will en tirely disappear. In one of these positions a smaller circu lar disk in the center of the star will be a certain color (red for instance) while in the other position it will exhibit the complementary color. This effect is obtained by placing the principal section of the small central disk 22f° from that of the other films of selenite which form the star. The rule to ascertain the time by this instrument is as follow: The tube must be turned round by the hand of the observer until the color star entirely disappears while the disk in the center re mains red ; the hand will then point accurately to the hour. The accuracy with which the solar time may be indicated by change in the plane corresponds with four minutes of solar ime. 'The instrument may be furnished with a graduated quadrant for the purpose of adapting it to any latitude ; but if It be intended to be fixed in any locality it may be perma nently adjusted to the proper polar elevation and the expense useful to adjust it accurately.
Size: 3335px × 4625px
Location:
Photo credit: © Corantos / Alamy / Afripics
License: Licensed
Model Released: No
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