Principles of Optometry; An Illustrated Text Book with Questions

Principles of Optometry; An Illustrated Text Book with Questions

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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. 1903 edition. Excerpt: ...is from C to lens B and then to D, and from F to lens B and then to H. The rays only seem to come from A. ' Suppose now that the object which is the source of light be brought nearer so that it is at A in Fig. 40, which is a duplicate of Fig. 39. Then the same thing will happen again. The rays of light will diverge, this being the effect of minus lenses, and the virtual image will be found still nearer to the lens at F. It is therefore evident that A and the distant object of Fig. 39 were not conjugate and that with minus lenses no matter how far or near the object may be the image is always on the same side of the lens and nearer to it, and that this image is always a virtual one. The nearer the object to the minus lens the nearer must the virtual image be. There is no aerial image at the focus of a minus lens, and this can be proved by trying to catch an image at that point with a white screen, which only results in cutting the light off from the lens. The relation of size of object and of image is the same, as is the case with plus lenses; that is, the size of one is to the size of the other as the distance of one from the lens is to the distance of the other from the lens. For example, if the object is twenty feet distant and the image is found at eight inches, then the diameter-of the object will be thirty times that of the image. The location of the virtual focus of a minus lens may be plotted out in a manner similar to that employed in finding the conjugate ' focus of a plus lens. From some point of the object not on the principal axis of the lens lay off a straight line which will pass through the optical center of the minus lens. This will be a secondary axis and will suffer no refraction; also on some point of this axis all the...show more

Product details

  • Paperback | 30 pages
  • 189 x 246 x 2mm | 73g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236816889
  • 9781236816887