Modern optical instruments and their construction / by Henry Orford.

  • Orford, Henry.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Modern optical instruments and their construction / by Henry Orford. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    of at F, a clear image would be formed of an object at o b without any effect of accommodation, whilst objects farther off would be focussed in front of the retina. This state, in which the posterior part of the eyeball is too long, so that, with the accommodation at rest, the retina lies at the conjugate focus of an object at a comparatively small distance, is called myopia. In Fig. 25, the inner line at R is the retina, and F the Fig. 25. principal focus of the lens system. Ra}^s emerging from R will, on leaving the eye, be convergent, and, meeting at the conjugate focus R', will form a clear image in the air; conversely, an object at R' will form a distinct image on the retina. The image of every object at a greater distance than R' will be formed more or less in front of R, and every such object must be indistinct. But objects nearer than R' will be seen clearly by accommo- dating just as in the normal eye, Fig. 23. The distance of r (R', Fig. 25) from the eye will depend on the distance of its conjugate focus R—that is, on the amount of the elongation of the eye. The greater the distance of R beyond F the less will be the conjugate focus R', and the more indistinct distant objects will become. If the elongation of the eye be very slight, R nearly coinciding