Annals of the Astronomical Observatory of Harvard College Volume 8

Annals of the Astronomical Observatory of Harvard College Volume 8

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1876 edition. Excerpt: ...or beyond an eye-piece used to enlarge the image. The second method has been often employed, and was generally adopted by other observers of the eclipse of 1869; but it seemed to me open to the following objections: I especially wished to obtain a good photograph of the corona, and apprehended that the fainter parts of its light would be too much enfeebled in their passage through the lenses of the eye-piece to produce a satisfactory picture. Moreover, the use of an eye-piece to enlarge the image also distorts it to an extent which it is difficult to determine accurately, and increases the difliculty of providing a fixed line of reference for such measurements as I wished to have made of the photographs I might obtain. Accordingly, after a few trials of both methods, I decided to have the photographs of the eclipse taken in the principal focus of the object-glass. " A second question, relating to the method of exposing the photographic plate, was also to be considered. At first, some experiments were tried with slides having openings with rounded ends or sides, under the impression that the exposure could thus be made more uniform. But experience and a more attentive consideration of the subject soon showed that the opening should have straight edges; for if we suppose two lines drawn through the centre of the image, one in the direction of the motion of the slide, and the other lat right angles with the first, the parts of the picture lying along one of these lines will be exposed differently from those lying along the other by a slide having an opening with curved edges. Of course, when the slide is placed before the object-glass, this matter becomes unimportant; but such an arrangement would be inconvenient in telescopes of the...show more

Product details

  • Paperback | 38 pages
  • 189 x 246 x 2mm | 86g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236751574
  • 9781236751577