Engineers' Surveying Instruments

Engineers' Surveying Instruments

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1892 edition. Excerpt: ...ROD. In Fig. 54 let a and b represent the stadia hairs; i the distance between them; s the distance, /, on the rod intercepted between the hairs; f the principal focal distance of the objective; e a point at a distance f-in front of the optical center of the objective, that is, e is the principal focus of the objective; c the distance from the plumb-line of the instrument to the optical center of the objective; y the distance from the outer focus, e, to the rod; and D the distance from the instrument to the rod. For convenience in printing represent. by k. From the principles of optics, we know that all rays of light which pass through e are parallel to each other after emerging from the objective. Therefore there is some point q, which will emit a single ray of light that will pass through e, and, after traversing the objective will strike the cross hair a. If the telescope is focused for the point q, the objective will bring all rays emitted by q to a focus at a; and hence it is immaterial whether we consider the real course of the rays, or assume that all the light from q passes along the line qea. Similarly we may assume that all the rays from / pass along the line peb. From Fig. 54 we easily get s: y:: i: f, from which y = 4s=ks (1) Notice that k, = 4, is a constant coefficient peculiar to each instrument, and also that the intercept.r on the rod varies asjy--the distance of the rod from the outer focus of the objective. These relations may be seen directly from Fig. 54. Since the two rays from / and q are parallel after entering the telescope, it is immaterial where the cross hairs are; and, therefore, the distance of the rod from e is always proportional to the intercept s. In other words, the intercept on the rod is proportional to thshow more

Product details

  • Paperback | 96 pages
  • 189 x 246 x 5mm | 186g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236509536
  • 9781236509536