**7. Negative lens**

pencils to the observer's pupil, their direction must be changed (Figure 6). This requires a fairly large lens somewhere between the patient's and the observer's eye. This principle was introduced by Ruete[29]in 1852 and is called indirect ophthalmoscopy to differentiate it from the first method, in which the light traveled in a straight, direct path from the patient's

**Figure 5.** Limited field of view in the direct method. Peripheral pencils of light do not reach the observer's pupil.

**Figure 6.** Extended field of view in the indirect method. The ophthalmoscopy lens redirects peripheral pencils of light

The use of the intermediate lens has several important implications that make indirect oph‐

The primary purpose of the ophthalmoscopy lens is to bend pencils of light toward the ob‐ server's pupil. Figure 3 also demonstrates one of the most characteristic side effects of this arrangement: Compared with the image in direct ophthalmoscopy, the orientation of the im‐ age on the observer's retina is inverted. For the novice, this often causes confusion in locali‐ zation and orientation. Figure 3 further shows that in this arrangement the patient's pupil is imaged in the pupillary plane of the observer. In optical terms the pupils are in conjugate

The most important changes are related to the change from candle light to gas light, to exter‐

Although the older generation found it difficult to adapt to the new instrument, the younger generation did so eagerly. One of them was Eduard von Jaeger (1828 to 1884) from Vienna,

thalmoscopy more complicated than direct ophthalmoscopy.

nal electric light and, finally, to built-in electric light sources.[31]

eye to the observer.

278 Glaucoma - Basic and Clinical Aspects

toward the observer.

planes.

A negative lens placed in front of the objective of the microscope can move the microscope focus to infinity. The practical application of this principle was worked out by Hruby[33], [34]of Vienna (1942) with a lens known as the Hruby lens.

The optical principle is best understood if the lens is considered in conjunction with the eye, rather than as a part of the microscope. Parallel rays emerging from an emmetropic eye are made divergent by the Hruby lens and seem to arise from the posterior focal plane of that lens (Figure 7A.) For a -50-D lens, this would be 20 mm behind the lens (the usual Hruby lens is -55 D). The slit-lamp microscope is thus looking at a virtual image of the fundus in a plane somewhere in the anterior segment and must be moved a little closer to the patient than it would be for the regular external examination.

To estimate the field of view in this method, it may be assumed that only rays emerging par‐ allel to the axis will reach the objective of the microscope and the observer's eye. When emerging from the eye, these rays must have been aimed at the anterior focal point of the Hruby lens. (Figure 7B), in which these rays are traced back to the retina, shows that the field of view (a) is proportional to the pupillary diameter as seen from the anterior focal point of the lens. This field is of the same order of magnitude as the field in direct ophthal‐ moscopy; it is largest when the lens is closest to the eye.

The refractive power of the cornea is eliminated in the contact lens. The only effective refrac‐ tive element left would seem to be the far less powerful crystalline lens. The retina is situat‐ ed well within the focal length of this lens, and the crystalline lens will therefore form a virtual image of the fundus (F) in a plane (F') behind the globe. How can the microscope fo‐ cus on an image that far back? We overlooked one other refracting surface: the plano front surface of the contact lens. F' is seen through plastic and vitreous. To the observer in air F' appears at F", through the same effect that makes a swimming pool appear shallower than it is. Because of this, the microscope again must focus on a plane inside the globe. As with the

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Thus, contact lens fundus microscopy extends our range of examination methods to details

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 high-

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

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 two-

With a 60-D lens the aerial image is as large as the fundus; thus the magnification is approxi‐

Can the field of view be widened even further? This is possible by using a contact lens of very high plus power with some additional optical tricks.Figure 9 illustrates the Roden‐

The unit contains a high plus contact lens, which forms an inverted fundus image (F') locat‐

In this arrangement, as in the previous example of a high myope (Figure 10), the imageforming and field-widening functions of the ophthalmoscopy lens are separated again. The contact lens forms the image; the spherical element serves to flatten the image and to redi‐

mately equal to the microscope magnification (similar to that with the Hruby lens).

slit lamps is that they cannot be pulled back far enough to observe this image.

Hruby lens, magnification is largely determined by the microscope.

beyond the reach of ordinary direct ophthalmoscopy.

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

disc diameter field of the Hruby lens.

**10. Contact lens for the indirect method**

ed inside a second, spherical glass element.

stockPanfunduscope, based on a design by Schiegel.[37]

power plus lens.

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

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. Limitation to the posterior pole is a disadvantage.

#### **8. Contact lens**

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

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

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

The refractive power of the cornea is eliminated in the contact lens. The only effective refrac‐ tive element left would seem to be the far less powerful crystalline lens. The retina is situat‐ ed well within the focal length of this lens, and the crystalline lens will therefore form a virtual image of the fundus (F) in a plane (F') behind the globe. How can the microscope fo‐ cus on an image that far back? We overlooked one other refracting surface: the plano front surface of the contact lens. F' is seen through plastic and vitreous. To the observer in air F' appears at F", through the same effect that makes a swimming pool appear shallower than it is. Because of this, the microscope again must focus on a plane inside the globe. As with the Hruby lens, magnification is largely determined by the microscope.

Thus, contact lens fundus microscopy extends our range of examination methods to details beyond the reach of ordinary direct ophthalmoscopy.
