Ophthalmoscope

Ophthalmoscope. This instrument is used for the examination of the fundus or back part of the interior of the eye. On looking at the transparent cornea the details of the interior of the eye are not visible, for, inasmuch as the head of the person making the examination is interposed between the source of illumination and the objects lying at the back of the eye, there is not sufficient light returning from the latter to render them visible, and all that is seen is the darkness of the pupil. By means of a mirror perforated in the centre, light can be reflected into the eye, and at the same time the observing eye, placed behind the hole in the mirror, can be made to receive the returning light from the eye which is being examined. This principle was first utilised by Mr. Babbage, but Helmholtz, in 1851, was the first to realise its important applications. There are two methods of using an ophthalmoscope. In the indirect method a convex lens is interposed in front of the eye examined, and the examining eye, situated some two feet from the eye observed, views an inverted image of the interior of the latter. In the direct method the examining eye is placed within a few inches of the eye examined, suitable lenses are made to slide in front of the aperture in the mirror, to correct, if need be, any error of refraction in the eye of either the patient or operator, and an erect image of the fundus of the observed eye is presented for examination. The ophthalmoscope has two main uses; in the first place, it is of great importance in medicine, as by means of it diseased conditions affecting the fundus of the eye can be detected. Such conditions are often a valuable guide in the diagnosis of disease. Optic neuritis, retinitis, and choroiditis, diseases affecting the fundus and readily distinguished by the ophthalmoscope, are far from being of merely local importance. The second great use of the ophthalmoscope is in the detection of errors of refraction.