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

  • Ernest Edmund Maddox
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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    valuable in spectrum analysis, is a serious dis- advantage in the ophthalmological use of prisms, from the coloured margins it appears to give to objects, and the want of definition it occasions, just in proportion to the strength of the prism used. It is only fairly weak prisms, therefore, that can be clinically employed with much advantage. It is not often advisable to exceed 3° or 4°, though the limits depend on the visual acuity, and habits of observation, of the patient. In amblyopia from corneal nebulae, stronger prisms can be borne. The dispersive power of crown glass is so much less than of flint glass that it is invariably employed for clinical purposes. Achromatic prisms can be made by cementing together two prisms, one of crown, the other of flint glass, with the apex of one to the base of the other, since the ratio between the refractive and dispersive power is different in the two kinds of glass.* But though suggested long ago, such combinations have been precluded by their weight from clinical use. * The dispersive power of crown glass is to that of flint glass as 33 to 52; whereas the refractive power is about as 31 to 32, though it varies greatly with different specimens, and must be determined for each.
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