Ophthalmoscope

Ophthalmoscope, an instrument by which the interior of the eye can be examined. It was first invented in 1847 by Charles Babbage (q.v.); but, as unfortunately the ophthalmic surgeon to whom he showed it did not recognise its importance, he laid it aside without making it generally known; and its principle had to be rediscovered by Professor Helmholtz, to whom belongs the credit of bringing it before the medical and scientific world in 1851. The value of the instrument depends on the circumstance that by illuminating and examining an eye in the same direction its deeper parts can be rendered visible. All forms of ophthalmoscope are adaptations of this principle. The form now generally in use resembles more that of Babbage than that of Helmholtz. It consists of a concave mirror of about 10 inches focus, 1 to 3 inches in diameter, with a small hole in the centre, and certain lenses to use with it, the most important of them a separate convex lens of $2\backslash frac\{1\}\{2\}$ inches focus, and $1\backslash frac\{1\}\{2\}$ to 4 inches in diameter. Examination is facilitated by dilating the pupil of the observed eye with atropine; and for a complete examination this is often indispensable. The person whose eye is to be examined is seated in a darkened room, with a bright light—e.g. a good gas-burner—on a level with his eye by the side of his head. The observer sits opposite him, and placing the mirror close to his own eye, and about 18 inches from the eye to be examined, reflects the light upon the latter, while he looks at it through the hole. The pupil in a healthy eye appears of a bright red or orange instead of its usual deep black. In short-sighted and long-sighted eyes, but not in normal ones, the vessels of the retina, the entrance of the optic nerve, &c. can be more or less distinctly seen, and by their movements the deviation from the normal refraction can roughly be estimated. Opacities in the lens (Cataract, q.v.) or vitreous humour appear black, and are discovered by this method more certainly and easily than by any other. The details of the retina, choroid, &c. (or fundus) can be seen in two different ways. In the indirect method the observer, seated as above described, holds the 2½-inch convex lens about 3 inches from the eye under examination, between it and his own, when a clear real image of part of the fundus, inverted and magnified about four diameters, appears in the red light of the pupil. In the direct method the observing eye must be placed as close to the observed as the intervention of the mirror will allow, when a virtual image of a smaller part of the fundus is seen, but erect and magnified about fourteen diameters. The fundus appears of an orange or red colour, varying much in different individuals; the blood-vessels of the retina are seen as darker red lines coursing over it. The entrance of the optic nerve, commonly called the disc, from which these vessels diverge, appears as a round area of a much paler colour. The ophthalmoscope has revolutionised this department of medicine, as most of the deeper affections of the eye, particularly of the optic nerve, choroid, and retina, were before only recognisable after the eyeball was removed from the body. Some of these affections have, moreover, important relations to general diseases—e.g. Bright's disease, diabetes, syphilis, diseases of the brain and spinal cord—and general medicine has benefited accordingly. The ophthalmoscope has also much facilitated the discovery and correction of errors of refraction (short- and long-sightedness, Astigmatism, q.v.; and see under EYE).