The Students Guide to Diseases of the Eye

the examination of opacities of the cornea, changes in the appearance of the iris, alterations in the outline and area of the pupil from iritis, and opacities of the lens. Such an examination is to be made by routine in every case before using the ophthalmoscope. We require a somewhat darkened room, a convex lens of two or three inches focal length (one of the large ophthalmoscope lenses), and a bright, naked lamp-flame. The patient is seated with his face towards the light, which is at about 2' distance. The lens, held between the finger and thumb, is used like a burning- glass, being placed at about its own focal length from the patient's cornea and in the line of the light, so as to throw a bright pencil of light on the front of the eye at an angle with the observer's line of sight. Thus all the superficial media and structures of the eye can be successively examined under strong illumination, the distance of the lens being varied a little, according as its focus is required to fall on the cornea, the iris, or the anterior or posterior surface of the crystalline lens (Fig. 27). By varying the position of the light and of the patient's eye, making him look up, down, and to each side, we can examine all parts of the corneal surface, of the iris, of the pupillaryarea (i.e. the anterior capsule of the lens), and of the lens-substance. If the light be thrown at a very acute angle on the cornea or lens, opacities are much more visible than if it fall almost pe...