Visibility of transparent objects in the eye by retroillumination.

Retroillumination is performed at the slit-lamp with the binocular microscope focused upon the object which is illuminated from behind by the slit-lamp beam reflected off some structure posterior to the object (Vogt, I930). Objects in the cornea and lens are observed in this way and the reflectors used are the iris, the lens, or the fundus. The use of the ophthalmoscope for retroillumination of the cornea is also described (Chandler, I945). Objects are visible because they interfere with the emergent light from the eye by obstruction, refraction, respersion (Berliner, 1943), and reflection. Transparent objects are visible by retroillumination when the refractive index of the object is above or below that of the surrounding medium. The property mainly responsible is refraction, but reflection also plays some part. The observed appearance changes with the direction of the retroillumination and with the refractive nature of the object. From the appearance of an object under varying retroillumination conditions, conclusions can be drawn about the refractive nature of the object. Although a variety of combinations of shapes and refractive indices are possible in the object, these can be resolved into only two observable effects: (i) The object acting as a diverging refractor (lens or cylinder). (2) The object acting as a converging refractor (lens or cylinder). Clinical examples are shown in macrophotographs made by the technique previously described (Brown, I970). Photographs are shown for comparison of a diverging lens (plano-concave) and of a converging lens (plano-convex) (Fig. 2). A model has been constructed to resemble objects found in the eye. The model has two spherical objects imbedded in a clear medium (gelatin): one object of low refractive index (air) which acts as a diverging lens, and one of high refractive index (liquid paraffin) which acts as a converging lens. Photographs are shown of the model under varying conditions of illumination (Figs 3, 4, and 5).