A contribution to ophthalmoscopy / by W. Laidlaw Purves.

  • Purves, William Laidlaw, -1917.
Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: A contribution to ophthalmoscopy / by W. Laidlaw Purves. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    same way as Stokes’ lens, you can obtain all the different degrees of convexity or concavity from 0 to l-5th, e.g. when the axis of the concave half corresponds to the axis of the convex half you have a Stokes at the zero point, and it acts as a Stokes when these two halves are rotated on each other; bnt when either the concave rotates on the other concave, or the convex on the other convex, you obtain an increase of the action as a con- cave or convex lens, and pass on from 1-10th to l-5th, according as the lenses rotate on each other. This used with or without a slit, gives a cylindrical with a spheri- cal action to a certain extent, the cylindrical action being less the narrower the slit. The different convex, cylindrical, or concave cylindrical lenses, may be used in the same way, and thus obtain by a few cylindrical lenses an amount of spherical action extending from the single to the combined power of the lenses employed. If the instrument is used by patients, it is better to limit them to a certain cpiadrant of the circle, before which the glasses can be revolved in any combination desired, the limit being made by a diaphragm. Having determined the refraction, the condition of the retina may be viewed by the same instrument (Stokes lens), but this will require a motion of the lens on its axis and of the indicator. And as some may possibly prefer, knowing now the glass required to give a distinct view of the fundus, to use a spherical lens in the usual manner, and so obtain a better view of the relations of the different portions of the Fundus to each other, I propose to construct an Ophthalmoscope, a combination of the modifications of those from Helm- holtz, Loring, and Schulek, with the addition of a gra-