The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox.

  • Maddox, Ernest E. (Ernest Edmund), 1860-1933.
Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: The clinical use of prisms : and the decentering of lenses / by Ernest E. Maddox. 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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    have a deviating angle equal to its refracting angle. But since for clinical purposes we never approach anything like this strength, and the smallest prism in the figure represents about the largest we pre- scribe, we have never to do with a deviating angle much more than half the refracting angle. The same figure shews us how easily we may find the deviating angle of any prism by construction. Draw any circle about a point O, and take any point A outside the circle, such that the distance A 0 shall be to the radius of the circle in the ratio of the refractive index of the glass. Thus, with crown glass, since its average refractive index is 1-54, if we make the radius one decimetre we shall make the distance A 6^=1 54 decimetres. Then with A as apex, draw the prism so that its base be shall be a chord of the circle, and its refract- ing angle A shall be bisected by a straight line between A and O. Then lines drawn from the centre {O) to the extremities of the base, enclose the deviating angle required. With weak prisms, there will be only a small error if we express the ratio of the deviating angle to the angle of refraction, by the decimal figures in the index of refraction of the glass. Thus, if the glass have an index of 1*5, the deviating angle is half (-5) of the refracting angle. If the index be the usual one of 1-54, the deviating angle is rather more than half ('54), and so on. For exact formulae,