The human eye : its optical construction popularly explained / by R.E. Dudgeon.

  • Robert Ellis Dudgeon
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: The human eye : its optical construction popularly explained / by R.E. Dudgeon. Source: Wellcome Collection.

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    3. A lens is a transparent body of peculiar shape, which possesses the propertj'- of refracting rays of light passing through it, that is to say, of deflecting them in certain directions, according to its shape and refractive power. 4, In order to understand the nature and properties of a lens, it will be necessary to explain what is meant by the refraction of light. When a ray of light from a transparent rnedium of a certain refractive power impinges perpendicularly on a transparent medium of a different refractive power, it passes through without deflexion, but if it im- pinges on this second medium obliquely, it is deflected from its original course and proceeds onward through the second medium in a new direction. The amount of divergence from its original course will depend on two things. First, the degree of obliquity with which the ray strikes the second medium; second, the difference betwixt the refractive power of the first and the second medium.* * Refraction is now usually explained to be caused by a retardation of the velocity of the vibrations of the ray of light, by the obstacles presented to its free passage through a so-called refractive medium. The phenomena of the spectrum are held to be due to this retarding effect acting with more or less power on the different coloured rays of which white light is composed. I have not thought it desirable or necessary to enter into fuller details on the subject of the cause of refraction. We have only to do here with the effects.
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