**9. "Indirect" slit-lamp microscopy**

**Figure 7.** Hruby lens. A. The fundus image (F') is formed in the posterior focal plane of the lens. B. The field of view is

With the lens close to the cornea, the fundus image will be close to the fundus plane and approximately actual size. The magnification to the observer is thus largely determined by the magnification of the microscope. At 16×, the magnification is about equal to that of direct ophthalmoscopy; at higher settings, the magnification is greater. Binocular viewing and slit illumination are advantages over direct ophthalmoscopy, even at similar magnification.

When the Hruby lens is moved progressively closer to the eye, it will eventually touch the cornea and become a contact lens. If the curvature of the posterior lens surface equals the curvature of the anterior corneal surface, the image formation will not change, but two re‐

The use of a contact lens for fundus examination was perfected by Goldmann[35]of Berne, Switzerland (1938). His contact lens is known for the three mirrors incorporated in it. These mirrors positioned at different angles make it possible to examine the peripheral retina with

**Figure 8.** Three mirror contact lens by Goldmann. Two of the three mirrors are shown. They allow visualization of dif‐

proportional to the size of the pupil as seen from the anterior focal point of the lens.

flecting surfaces will be eliminated, and image clarity will increase.

little manipulation of the patient's eye or of the microscope axis (Figure 8).

Limitation to the posterior pole is a disadvantage.

**8. Contact lens**

280 Glaucoma - Basic and Clinical Aspects

ferent parts of the fundus.

The use of the Hruby lens and Goldmann contact lens is comparable to direct ophthalmo‐ scopy, because no real intermediate image is formed. The equivalent of indirect ophthalmo‐ scopy can be achieved by focusing the microscope on the real image formed by a highpower plus lens.

El Bayadi[36]introduced the use of a +60-D lens for this purpose. The inverted image formed by this lens is situated 16 mm (0.0167 m) in front of it. A practical problem with some older slit lamps is that they cannot be pulled back far enough to observe this image.

Compared with the Hruby (-55 D) lens, the El Bayadi (+60 D) lens offers the same major ad‐ vantage as does indirect ophthalmoscopy: a larger field of view. With proper placement of the lens, the field is about six disc diameters (40 degrees), compared with the one- or twodisc diameter field of the Hruby lens.

With a 60-D lens the aerial image is as large as the fundus; thus the magnification is approxi‐ mately equal to the microscope magnification (similar to that with the Hruby lens).
