**5. Ophthalmoscopy**

Ophthalmoscopy, the most important single invention in ophthalmology, that had shaped its evolution, was introduced by Hermann von Helmholtz in December of 1850.[20], [21]However, Jan Purkinje (known for the Purkinje images) had described the complete technique and published it in Latin in 1823,[22]but his audience apparently was not yet ready and his publication went unnoticed. A quarter of a century later, however, the situa‐ tion changed.

Helmholtz' monograph on ophthalmoscopy was published in 1851 and soon was widely cir‐ culated. The next year there were several important improvements contributed by other workers. Rekoss,[28]von Helmholtz' instrument maker, added two movable disks with lens‐ es for easier focusing. Epkens, working with Donders in Holland,[27] introduced a perforat‐ ed mirror for increased illumination. Ruete[29] in Germany did the same and also developed the indirect method of ophthalmoscopy. With these basic components in place, future generations provided technical improvements. In 1913, Landolt[30] listed 200 differ‐

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If the patient's fundus is properly illuminated, the field of view is limited by the most obli‐ que pencil of light that can still pass from the patient's pupil to the observer's pupil (Figure 4.). In direct ophthalmoscopy the retinal point that corresponds to this beam can be found by constructing an auxiliary ray through the nodal point of the eye.[30] The point farthest from the centerline of view that can still be seen is determined by the angle α, that is, the

**Figure 4.** Field limits in direct ophthalmoscopy. The maximum field of view is determined by the most oblique pencil

Angle α, and therefore the field of view, is increased when the patient's or the observer's pu‐

The more peripheral pencils of light use ever-smaller parts of each pupil. This means that, even if the patient's fundus is uniformly illuminated, the luminosity of the fundus image gradually decreases toward the periphery, so that there is no sharp limitation to the field of vision. In practice, therefore, the effective field of vision is determined by the illuminating system not by the viewing system. Most ophthalmoscopes project a beam of light of about

Even with appropriate illumination, direct ophthalmoscopy has a small field of view (Figure 5.) shows that of four points in the fundus, points one and four cannot be seen because pen‐ cils of light emanating from these points diverge beyond the observer's pupil. To bring these

angle between this oblique pencil and the common optical axis of the eyes.

of rays (shaded) that can still pass from one pupil to the other.

one disc diameter.

**5.2. Indirect ophthalmoscopy**

pil is dilated or when the eyes are brought more closely together.

ent types of ophthalmoscopes.

**5.1. Direct ophthalmoscopy**

The ophthalmoscope was not based on any radically new concepts. Rather, it combined the appropriate application of various known principles with recognition of its potential impact and presentation to an appropriate audience. Under the leadership of men like Bowman in London, Donders in Holland, and von Graefe and von Helmholtz in Germany, ophthalmol‐ ogy emerged as the first organ-based specialty in medicine.

*Bowman (1816 to 1892) is known for Bowman's membrane and for his work in anatomy and histology.*

*Donders (1818 to 1889) clarified the principles of refraction and accommodation (1864) and defined visual acuity as a measurable quantity. His coworker Snellen developed the Snellen chart.*

*In Berlin, Albrecht von Graefe (1828 to 1870) was a leader in stimulating the clinical application of new techniques and the careful documentation of new findings. He is remembered for Graefe's knife and Graefe's Archives (1854) (one of the first ophthalmic journals), and he founded the German Oph‐ thalmological Society (Heidelberg, 1857).*

Several workers had tried to visualize the inside of the eye but had fallen short of putting it all together. Kussmaul (known for "Kussmaul'sairhunger") described the imaging princi‐ ples in a thesis in 1845[23]but failed to solve the illumination problem. Cumming[24](1846) in England and Brücke[25](1847) in Germany had shown that a reflection from the fundus could be obtained by bringing the light source in line with the observer, but they failed to solve the imaging problem. Babbage,[26]the English mathematician, reportedly constructed an ophthalmoscope in 1847, but his ophthalmologist friend did not recognize the impor‐ tance and did not publish it until 1854, when von Helmholtz' instrument was well known.

In the fall of 1850, von Helmholtz tried to demonstrate the inside of the eye to the students in his physiology class. On December 6, he presented his findings to the Berlin Physical Soci‐ ety[20]; on December 17, he wrote to his father[27]:

*"I have made a discovery during my lectures on the Physiology of the Sense-organs, which was so obvious, requiring, moreover, no knowledge beyond the optics I learned at the Gymnasium, that it seems almost ludicrous that I and others should have been so slow as not to see it…. Till now a whole series of most important eye-diseases, known collectively as black cataract, has been terra incognita…. My discovery makes the minute investigation of the internal structures of the eye a possibility. I have announced this very precious egg of Columbus to the Physical Society at Berlin, as my property, and am now having an improved and more convenient instrument constructed to replace my pasteboard affair…"*

Helmholtz' monograph on ophthalmoscopy was published in 1851 and soon was widely cir‐ culated. The next year there were several important improvements contributed by other workers. Rekoss,[28]von Helmholtz' instrument maker, added two movable disks with lens‐ es for easier focusing. Epkens, working with Donders in Holland,[27] introduced a perforat‐ ed mirror for increased illumination. Ruete[29] in Germany did the same and also developed the indirect method of ophthalmoscopy. With these basic components in place, future generations provided technical improvements. In 1913, Landolt[30] listed 200 differ‐ ent types of ophthalmoscopes.

#### **5.1. Direct ophthalmoscopy**

**5. Ophthalmoscopy**

276 Glaucoma - Basic and Clinical Aspects

tion changed.

*histology.*

*affair…"*

Ophthalmoscopy, the most important single invention in ophthalmology, that had shaped its evolution, was introduced by Hermann von Helmholtz in December of 1850.[20], [21]However, Jan Purkinje (known for the Purkinje images) had described the complete technique and published it in Latin in 1823,[22]but his audience apparently was not yet ready and his publication went unnoticed. A quarter of a century later, however, the situa‐

The ophthalmoscope was not based on any radically new concepts. Rather, it combined the appropriate application of various known principles with recognition of its potential impact and presentation to an appropriate audience. Under the leadership of men like Bowman in London, Donders in Holland, and von Graefe and von Helmholtz in Germany, ophthalmol‐

*Bowman (1816 to 1892) is known for Bowman's membrane and for his work in anatomy and*

*Donders (1818 to 1889) clarified the principles of refraction and accommodation (1864) and defined*

*In Berlin, Albrecht von Graefe (1828 to 1870) was a leader in stimulating the clinical application of new techniques and the careful documentation of new findings. He is remembered for Graefe's knife and Graefe's Archives (1854) (one of the first ophthalmic journals), and he founded the German Oph‐*

Several workers had tried to visualize the inside of the eye but had fallen short of putting it all together. Kussmaul (known for "Kussmaul'sairhunger") described the imaging princi‐ ples in a thesis in 1845[23]but failed to solve the illumination problem. Cumming[24](1846) in England and Brücke[25](1847) in Germany had shown that a reflection from the fundus could be obtained by bringing the light source in line with the observer, but they failed to solve the imaging problem. Babbage,[26]the English mathematician, reportedly constructed an ophthalmoscope in 1847, but his ophthalmologist friend did not recognize the impor‐ tance and did not publish it until 1854, when von Helmholtz' instrument was well known.

In the fall of 1850, von Helmholtz tried to demonstrate the inside of the eye to the students in his physiology class. On December 6, he presented his findings to the Berlin Physical Soci‐

*"I have made a discovery during my lectures on the Physiology of the Sense-organs, which was so obvious, requiring, moreover, no knowledge beyond the optics I learned at the Gymnasium, that it seems almost ludicrous that I and others should have been so slow as not to see it…. Till now a whole series of most important eye-diseases, known collectively as black cataract, has been terra incognita…. My discovery makes the minute investigation of the internal structures of the eye a possibility. I have announced this very precious egg of Columbus to the Physical Society at Berlin, as my property, and am now having an improved and more convenient instrument constructed to replace my pasteboard*

*visual acuity as a measurable quantity. His coworker Snellen developed the Snellen chart.*

ogy emerged as the first organ-based specialty in medicine.

*thalmological Society (Heidelberg, 1857).*

ety[20]; on December 17, he wrote to his father[27]:

If the patient's fundus is properly illuminated, the field of view is limited by the most obli‐ que pencil of light that can still pass from the patient's pupil to the observer's pupil (Figure 4.). In direct ophthalmoscopy the retinal point that corresponds to this beam can be found by constructing an auxiliary ray through the nodal point of the eye.[30] The point farthest from the centerline of view that can still be seen is determined by the angle α, that is, the angle between this oblique pencil and the common optical axis of the eyes.

**Figure 4.** Field limits in direct ophthalmoscopy. The maximum field of view is determined by the most oblique pencil of rays (shaded) that can still pass from one pupil to the other.

Angle α, and therefore the field of view, is increased when the patient's or the observer's pu‐ pil is dilated or when the eyes are brought more closely together.

The more peripheral pencils of light use ever-smaller parts of each pupil. This means that, even if the patient's fundus is uniformly illuminated, the luminosity of the fundus image gradually decreases toward the periphery, so that there is no sharp limitation to the field of vision. In practice, therefore, the effective field of vision is determined by the illuminating system not by the viewing system. Most ophthalmoscopes project a beam of light of about one disc diameter.

#### **5.2. Indirect ophthalmoscopy**

Even with appropriate illumination, direct ophthalmoscopy has a small field of view (Figure 5.) shows that of four points in the fundus, points one and four cannot be seen because pen‐ cils of light emanating from these points diverge beyond the observer's pupil. To bring these 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 eye to the observer.

best known for his print samples that were based on the print catalogue of the Vienna State Printing House. He was the son of a well-known ophthalmologist and an artistically gifted mother. In 1855, at the age of 27, he published his first atlas; he continued to add to his col‐

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279

Although not generally considered as a method of ophthalmoscopy, fundus examination with the slit lamp offers an important addition to the traditional methods of direct and indi‐ rect ophthalmoscopy. It offers the advantage of high-power magnification through the mi‐ croscope and flexible illumination with the slit-lamp beam. With appropriate contact lenses, it can offer higher magnification than direct ophthalmoscopy and a field several times wider than indirect ophthalmoscopy. These methods have become particularly important in com‐

Because the slit-lamp microscope has a fixed focus on a plane approximately 10 cm in front of the objective and because the image of the fundus of an emmetropic eye appears at infini‐ ty, the fundus cannot be visualized without the help of additional lenses. There are several

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],

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

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‐

lection of authoritative fundus paintings until his death in 1884.[32]

**6. Slit-lamp examination of the fundus**

[34]of Vienna (1942) with a lens known as the Hruby lens.

than it would be for the regular external examination.

moscopy; it is largest when the lens is closest to the eye.

bination with laser treatment.

options.

**7. Negative lens**

**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 toward the observer.

The use of the intermediate lens has several important implications that make indirect oph‐ thalmoscopy more complicated than direct ophthalmoscopy.

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 planes.

The most important changes are related to the change from candle light to gas light, to exter‐ nal electric light and, finally, to built-in electric light sources.[31]

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, best known for his print samples that were based on the print catalogue of the Vienna State Printing House. He was the son of a well-known ophthalmologist and an artistically gifted mother. In 1855, at the age of 27, he published his first atlas; he continued to add to his col‐ lection of authoritative fundus paintings until his death in 1884.[32]
