Elementary ophthalmic optics : including ophthalmoscopy and retinoscopy / by J. Herbert Parsons.

  • John Parsons
Date:
1901
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Licence: In copyright

Credit: Elementary ophthalmic optics : including ophthalmoscopy and retinoscopy / by J. Herbert Parsons. 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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    pencil meet in a point, this point is called the focus of the pencil. If an opaque body lies in the area of illumination it will cut off certain of the rays of light. If B be a luminous point, and A an opaque body, then a shadow, EF, will be cast upon the screen, S. As, however, most luminous bodies are of appreciable size, if CD be a luminous body, then A will cause a shadow of the light coming from C at GF, and a shadow of the light coming from D at EH, and similarly for all intermediate points. The areas EG, HF will continue to receive rays of light from CD, and will therefore still be partially illuminated. GH is called the umbra or total shadow, and EG, HF, the penumbra or partial shadow. For the explanation of the phenomena of light, physicists have introduced the conception of an hypothetical medium, ether, the peculiar undulations
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