Lectures on diseases of the eye / by N.C. Macnamara. Part I., Referring principally to those affections requiring the aid of the ophthalmoscope for their diagnosis.

  • Macnamara, Nottidge Charles, 1832-1918.
Date:
1866
Catalogue details

Licence: Public Domain Mark

Credit: Lectures on diseases of the eye / by N.C. Macnamara. Part I., Referring principally to those affections requiring the aid of the ophthalmoscope for their diagnosis. 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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    by the lens L, will be united at a and /3' respec- tively, at the distance, approximately, from L of q its principal focus. A real, inverted, and magnified image a (3'} of a j3, will thus be formed; and this will be distinctly visible by A at a distance of twelve or fourteen inches, the rays diverging from a being brought to a focus on the retina at a, and those from p at ft. The indirect method of examination is more gene- rally applicable for ophthalmoscopic purposes than the direct, for the following reasons :— 1st.—A clear and distinct image of a vessel, or any other small object on the retina, may be seen at a distance of twelve or fourteen inches. 2nd.—Although the magnification is less, the field of view is larger, and a considerable portion of the fundus of the eye being visible at once, its several parts may be studied and compared in their relative positions at a glance. 3rd.—In brightness and clearness, the image seen by the indirect, greatly exceeds that observed by the direct process of examination. With regard to the 6 est form of ophthalmoscope, for ordinary purposes I prefer the one invented by Coccius, and as most of you have been in the habit of using this instrument in the ophthalmic wards, you had better continue to do so. It consists of a small metallic surface coated with silver; in its centre is a funnel-shaped hole about the one-sixteenth of an inch in diameter, and a plate of blackened metal is made to