A general theory for the mensuration of the angle subtended by two objects, of which one is observed by rays after two reflections from plane surfaces, and the other by rays coming directly to the spectator's eye / [G. Atwood].

  • Atwood, G. (George), 1745-1807
Date:
[1781]
Catalogue details

Licence: Public Domain Mark

Credit: A general theory for the mensuration of the angle subtended by two objects, of which one is observed by rays after two reflections from plane surfaces, and the other by rays coming directly to the spectator's eye / [G. Atwood]. Source: Wellcome Collection.

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    -on the fubjeX, and ihall be happy if the refult of my in- quiries appears to merit the attention of the Royal Society. Art. i. The manner of taking an obi'ervation by two re- flexions unconfined to any particular cafe may be deferibed thus. Let C, B (fig. i.) reprefent two plane refleXing furfaces, inclined to a plane OPA at any given angle. Through any point of the refleXing furface C draw a line perpendicular to the plane OPA, and with the point where the line meets the plane as a centre (which muft here be reprefented by C) and any diftance CP, deferibe a circle OPA. The refleXing plane B always continuing fixed, let the refleXor C be moveable along with the radius CP as it revolves in the plane OPA round the centre C : the angular motion of the fipeculum C, referred to the circumference OPA, will be meafured by the arc which the radius CP deferibes, the inclination of the plane C to the plane of motion OPA being always the fame, and equal to that tof the fixed fpeculum B. 2. The two plane refleXors, B and C, being equally in* dined to the plane OPA, it follows, that during the motion of C there muft be fome point O in the circumference OAP, at which when CP arrives, the refleXor C will be parallel to the fixed refleXor B. 3. When the moveable radius which carries round the plane C is at any other pofition CP, let a ray flowing from a diftant objeX T impinge on the fpeculum C; let it be refleXed from thence in the dircXion CB, and being again refleXed at B in the direXion BG, let it be obferved by a 1'peXator’s eye at G; the image of T will appear fomewhere in the line GBS ; fuppofe that a ray flows from a diftant objeX S fituated in the line GB produced, and that this ray SG comes direXly to the fpeXator’s B eve