Ophthalmoscopy, Retinoscopy and Refraction

Ophthalmoscopy, Retinoscopy and Refraction

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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. 1922 edition. Excerpt: ...of illumination and a screen placed at the proper distance irom the lens. When the source of illumination is placed twenty feet or farther away, usually spoken of as infinity from the lens, the rays that leave it and enter the latter, do so practically parallel to each other; and, in order to secure a distant image, which will be inverted, of the source of illumination, the screen must be placed at a certain definite distance from the center of the lens. This point is called the principal focus of the lens and its distance from the center of the latter is the focal length. This last is made use of in computing the dioptric or refractive strength of the lens as described on page 127. Since the image in this case can be demonstrated as described above, it is spoken of as a positive or real image, and the point at which it occurs as a positive or real focus. Fig. 42. The Action of a Convex Fig. 43. The Action of a Concave Lens on Parallel Rays. (May.) Lens on Parallel Rays. (May.) Action Of Concave Lenses: Figure 43. When we apply the above principles to the concave lens, we secure the very different result illustrated in Figure 43.. The parallel rays entering the lens are caused to diverge and obviously can never come to a focus. However, if we place one eye in the path of these divergent rays, we are able to observe an erect reduced image of the source of illumination which is the result of the prolonging backward of the refracted divergent rays shown by the dotted lines in Figure 43; this image is located at the principal focus of the lens. Such an image which cannot be thrown on a screen and can be observed only through the concave lens producing it, is spoken of as a virtual image. Conjugate Foci: Another, but minor matter to be considered in...show more

Product details

  • Paperback | 32 pages
  • 189 x 246 x 2mm | 77g
  • Rarebooksclub.com
  • United States
  • English
  • black & white illustrations
  • 1236506707
  • 9781236506702