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The Ophthalmoscope in the Lifetime of Hermann von Helmholtz
C. Richard Keeler
Arch Ophthalmol. 2002;120:194-201.
INTRODUCTION
"In the whole history of medicine there is no more beautiful episode
than the invention of the ophthalmoscope, and physiology has few greater triumphs."
Thus wrote American ophthalmologist Edward Loring1
in the opening paragraph of his Textbook of Ophthalmology in 1892, 2 years before the death of Hermann von Helmholtz.
On the 150th anniversary of the invention or "discovery" of the ophthalmoscope
(Figure 1) by Helmholtz, we have
an opportunity, once again, to laud this outstanding physicist of the 19th
century, just as his peers did on the 10th, 50th, and centennial years of
this greatest of ophthalmological inventions.
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Figure 1. Early model of the Helmholtz ophthalmoscope,
1851.
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Prior to his invention, ophthalmologists could not view the posterior
section of the eye and struggled to explain certain classes of eye disease
in which there was a dimness or loss of vision. Suddenly in 1851, the world
of ophthalmology was taken by surprise, and a new epoch began. Immediately
after the discovery of the ophthalmoscope, men such as Albert von Graefe in
Berlin, Germany, Edward Jaeger in Vienna, Austria, and William Bowman in London,
England, started using it. Every look into the eye became a discovery.
In 1893 at age 72 years, a year before he died, Helmholtz went to America
to attend the World's Fair in Chicago, Ill, as a representative of the German
government. On his way back, he was invited to New York, NY, by Professor
Herman Knapp to inaugurate his new clinic before an audience of students and
members of the medical profession. Professor Knapp, a former pupil who had
begun studying medicine in Germany the very year that the ophthalmoscope was
invented, had moved to New York in 1868 from Heidelberg, where he had been
a professor for 3 years.
Helmholtz told the well-known story of how 43 years before, he realized
that he had solved the mystery of how to penetrate the black pupil of the
eye and view the fundus. In 1851, Helmholtz had published his Beschreibung eines Augen-Spiegels2,
which gave a full account of the optical principles involved. Later he proudly
told his father that he had received 18 orders for his instrument from throughout
Europe.
By the time of his death, a great number of different ophthalmoscopes
had appeared, many of them designed by the most eminent ophthalmologists of
the day. Loring, in the introduction to his textbook, states, "[A]lmost every
ophthalmologist has taken a hand in perfecting or at least altering the instrument,
and from the first I have, perhaps, done more than my share."1(p2) There were no less than 140 instruments on exhibition at a meeting hosted
by Harry Friedenwald and Casey Wood in Atlantic City, NJ, on the 50th anniversary
of the invention of the ophthalmoscope.3
Helmholtz did not readdress the construction of the ophthalmoscope but
instead became the greatest physicist and physiologist of the time. He had
demonstrated that there were 3 essential elements to the working of an ophthalmoscope:
a source of illumination, a reflecting surface to direct light toward the
eye, and a means of correcting an out-of-focus image on the fundus.
SOURCES OF ILLUMINATION
It was the lack of a strong, stable source of illumination that held
up the development of the ophthalmoscope in the 19th century.
Early users of the Helmholtz ophthalmoscope had to put up with a naked
flickering candle as a light source. During this first decade, the candle
was largely replaced by the oil lamp and then the paraffin-burning lamp.
Various valiant attempts were made at this time to allow the source
of illumination to follow the movements of the ophthalmoscope: the first by
Ricardus Ulrich in 1854 with his candleholder precariously attached to the
observation tube,4(p40) and the
second by Lionel Beale in 1869 with his built-in oil lamp.5(p97)
Late in the 18th century, Swiss-born physician and chemist Aimé
Argand had invented a device that was to evolve into the most common source
of illumination during the second half of the 19th century: the gas lamp.
In the ordinary oil lamp, combustion was not complete. Argand's improvement
was the replacement of the conventional wick with a ring. The flame became
a hollow cylinder with a current of air ascending through the inside so that
the burning surface was doubled. Argand's brother accidentally discovered
that a glass cylinder placed as a chimney over the flame steadied it, created
a draft, and allowed the flame to yield the maximum amount of light.6
The gravity-fed oil lamp was followed by the gas-burning lamp, which
worked on the same principle (Figure 2);
by 1869, this source of illumination had become the standard. This was fine
for examinations in a fixed location such as an ophthalmologist's office or
the hospital, but for domiciliary visits the portable candle or oil lamp,
such as the one devised by Joseph Priestley Smith of Birmingham, England,4(p270) was used well into the 20th century.
These lamps incorporated a reflecting mirror behind the candle and a strong
convex lens in front to condense the light.
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Figure 2. Nineteenth-century gravity-fed
oil lamp (left) and gas lamp (right).
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In 1879, Thomas Edison was working on the incandescent bulb. Six years
later William Dennett, a New York ophthalmologist, demonstrated this new technology
at the American Ophthalmological Society when he presented the first ophthalmoscope
that used an electric bulb.7 It was not a success,
mainly because of the unreliability and short life of the bulb.
The following year saw the emergence of 3 ophthalmoscope designs incorporating
electric bulbs. Like Dennett, Thomas Reid of Glasgow, Scotland, placed a bulb
inside the column of his instrument, but he used a prism instead of a mirror
to project the light. Although this model was shown at the 1886 meeting of
the Ophthalmological Society of the United Kingdom, it never went into production.8 Sir James McKenzie Davidson of Aberdeen, Scotland,
who was one of the early pioneers of the use of x-ray in ophthalmology, published
an article in the Lancet of January 1886 showing
a diagram of an electric ophthalmoscope.9 The
third electric ophthalmoscope was designed by Henry Juler of London, England.
Unlike Davidson's concept, Juler's instrument went into production.
Juler's design (Figure 3)
was the attachment of a light source to the outside of the ophthalmoscope
body, close to the mirror with the bulb pointing toward the center aperture.
Apart from excessive sight-hole flare, this instrument had the same problem
of a short bulb life. However, various models using this method became popular.
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Figure 3. Juler electric ophthalmoscope
showing the driving wheel (A) that moves the lenses around the track, the
lamp housing (B) containing the bulb, and the Le Clanché battery (C)
that powers the small lamp.
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METHODS OF REFLECTING LIGHT
Haynes Walton10 of St Mary's Hospital
in London wrote in his Practical Treatise on the Diseases
of the Eye in 1875, "The modes of reflecting light into the eye are
manifold. It is done by unsilvered and by silvered reflectors; by reflectors
alone and by reflectors in combination with lenses; by prisms also and by
lenses which are themselves silvered."
Helmholtz brilliantly devised a method of light reflection by placing
superimposed plates of plain glass at an angle to the incident light. In his
monograph,2 he was very precise about the angle
of incidence needed to obtain the maximum illumination when using either 1,
3, or 4 plates, with partial polarization of the distractingly bright corneal
reflex being achieved with 3 or more plates. His weak-light mirror, as it
was also known, had its advocates, especially those who wanted to detect slight
variations and shades of color in the fundus. However, the Helmholtz ophthalmoscope
could be used only for direct ophthalmoscopy, and the vogue for the indirect
method had caught on rapidly.
His glass plates method of light reflection was quickly superceded when
Epkens, an instrument maker in Holland, followed by others, used a plano-mirror
with an oval-shaped aperture cut out of the middle. Epkens then collaborated
with Frans Donders of Utrecht to produce several table-mounted model ophthalmoscopes.11(p32-37) Four years previously in 1847,
Charles Babbage, of calculating-machine fame, had used the same mirror design
in his prototype ophthalmoscope.12
The plano-mirror was an inefficient gatherer of light. In an attempt
to concentrate the source of illumination, Adolf Coccius in 1853 used a biconvex
condensing lens on an adjustable arm to focus the source of light onto the
plano-mirror (Figure 4). Although
his first model used a plano-mirror, later this became concave, further increasing
the concentration of the light.
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Figure 4. The first ophthalmoscope by Adolf
Coccius, with a plano-mirror and biconvex lens for focusing the light.
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In 1854, Professor Karl von Zehender constructed an instrument that
had an uncanny resemblance to the second model by Coccius,11(p42) but it did differ in 2 important respects: the mirror was convex
and was made of metal, not glass. Metal provided sharp edges to the sighthole
with no restrictive viewing canal, which often occurred when using thick glass
mirrors.
Professor G. Theodor Ruete of Leipsig, Germany, was the first to introduce
a concave mirror with aperture, in 1853. This method of reflecting light toward
the eye was to last the whole of Helmholtz's lifetime and well beyond. Ruete
used this concave focusing mirror in his new indirect ophthalmoscope (Figure 5), a method of ophthalmoscopy13 that Helmholtz had anticipated and that was to prove
of extraordinary importance in ophthalmology. Most ophthalmoscopes thereafter
accommodated mirrors and lenses, allowing the ophthalmologist to switch between
direct and indirect ophthalmoscopy on the same instrument.
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Figure 5. G. Theodor Ruete's indirect ophthalmoscope
using a concave mirror with aperture (3).
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In 1854, Edward Jaeger of Vienna, Austria, introduced an ophthalmoscope
with a choice of aperture mirrors with a 4- or 7-in focal length as well as
his version of Helmholtz's glass plates, each of which could be angled toward
the source of illumination.4(p102)
Jaeger thereby combined the principles of Helmholtz, Ruete, and others in
one rather complicated instrument.
A common problem in all ophthalmoscopes when tilting the mirror toward
the light was having to view the fundus obliquely through the correcting lens,
as the mirror was mounted flat against the Rekoss disc. This produced a significant
reduction in vision and shift of the image when viewing with higher-power
lenses.
John Couper of the Royal London Ophthalmic Hospital, Moorfields, overcame
this problem in 1875. He mounted his tiltable mirror on a vertical axis so
that the source of light could be received by the mirror, which was placed
left or right at an angle to the Rekoss disc. The observer was able to view
the fundus looking perpendicularly through whichever lens was appropriate.
Couper's first instrument, now in the Institute of Ophthalmology's collection
in London (Figure 6, left), may
be his own unique design. Couper's next model (Figure 6, right) had a more convenient and sophisticated method
of swinging the mirror left and right. His tilting mirror was a turning point
in the evolution of the ophthalmoscope.14
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Figure 6. John Couper's first ophthalmoscope,
with a slide-in Rekoss disc behind a tilting mirror (left). Right, second
model.
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Couper's idea was followed in 1876 by one from Oliver Wadsworth of Boston,
Mass, who designed an angled concave mirror mounted in a disc that could be
placed left or right on the head of a Loring ophthalmoscope.4(p346) This conveniently overcame the parallax in earlier instruments
and moved the sight-hole and the mirror aperture closer together, allowing
the observer to obtain a wider field of view of the fundus.
In the same year, Edward Loring went further than Couper and Wadsworth
and produced his vertically held, tilting, rectangular concave mirror (Figure 7). With the sides of a round mirror
cut off, a sufficient tilting angle could be achieved while keeping a minimal
distance between the sight-hole and mirror aperture. This mirror was extremely
simple to use in either the left or right position. Its construction became
very popular and was to be incorporated into many other designs during the
next 20 years.
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Figure 7. Loring ophthalmoscope with a rectangular
concave tilting mirror.
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Until 1882, the need to use mirrors of more than one focal length for
the direct and indirect method of ophthalmoscopy meant the removal of one
mirror for another. In that year, however, George Lindsay Johnson of London
introduced an ophthalmoscope (Figure 8) with 2 mirrors fixed to an arm that could be rotated to bring the appropriate
mirror into position in front of the sight-hole. The smaller mirror, with
a 3-in focal length, rotated around its own axis, and the larger mirror had
a focal length of 18 in. Later variations of 3 or even 4 mirrors, back to
back, were to dominate the more sophisticated devices such as the Morton ophthalmoscope
for the next 40 years.
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Figure 8. Lindsay Johnson ophthalmoscope
with 2 mirrors for direct and indirect ophthalmoscopy.
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METHODS OF CORRECTION
Shortly after Helmholtz produced his first ophthalmoscope, a major deficiency
became apparent. There was no convenient means of correcting for presbyopia
or refractive errors. The optics of the Helmholtz instrument produced a converging
image as the rays entered the observer's eye15;
this was the main reason that the early instruments used only concave lenses.
In 1852, Egbert Rekoss, a university machinist who had made Helmholtz's original
instrument, provided the essential breakthrough by adding 2 rotatable discs,
each containing a series of lenses.
Richard Liebreich, while in Berlin, Germany, at von Graefe's clinic
in 1855, designed a simple and very popular ophthalmoscope (Figure 9) with a clip mounted on an adjustable arm behind the mirror
for a series of unmounted convex and concave lenses. Liebreich was not the
first to use this idea; 2 years before, Coccius had done the same.4(p33)
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Figure 9. Miniature model of the Liebreich
ophthalmoscope complete with clip for holding correcting lenses and 2 indirect
condensing lenses.
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Jaeger's 1854 ophthalmoscope included 8 concave and 4 convex lenses,
with the chosen lens having to be individually inserted into an aperture at
the back of the instrument, a tedious and impractical procedure. In 1869,
a degree of sophistication crept in with Loring's first of several ophthalmoscope
designs. This instrument had 3 Rekoss discs, each with 8 lenses: one disc
contained concave lenses of moderate power, another had convex lenses of low
power, and a third was composed of high dioptric power, both convex and concave.
This increased battery of lenses allowed Loring to perform refractions as
well as ophthalmoscopy. The ophthalmoscope illustrated in Figure 10 has a fourth disc with a further combination of low convex
and concave lenses that could have been tailor-made for the operator.
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Figure 10. Loring ophthalmoscope with 4
different lens power combination Rekoss discs.
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In 1873, shortly after arriving in New York, Herman Knapp, a prolific
inventor of surgical and diagnostic instruments, devised an alternative and
more efficient way of introducing a wider range of lenses in an ophthalmoscope.16 He was especially interested in carrying out refractions
and wanted not only a wider range of lenses but smaller jumps in power. He
used 2 Rekoss discs, one with convex lenses and the other with concave lenses,
so that the bottom of one and the top of the other overlapped. Knapp had admired
Loring's 1869 model but wanted to avoid the loss of time and the tedious process
of assembly and disassembly for each patient.
Xavier Galezowski of Paris, France, who had invented his tubular indirect
ophthalmoscope in 1862, designed a very different ophthalmoscope (Figure 11) 20 years later in 1882. This
used a single Rekoss disc but with 2 concentric rings of lenses mounted within
it. The outer ring had 19 concave lenses, and the inner one had 13 convex
lenses. Pushing the disc up or pulling it down brought the chosen circle of
convex or concave lenses to the center of the viewing aperture. Although Loring
had shown the way in 1874, in his Rekoss disc, half of each of the concentric
circles had convex lenses, and the other half had concave lenses.4(p330)
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Figure 11. Galezowski ophthalmoscope (dual-mirror
side) with a Rekoss disc of 2 concentric rings of lenses in the down position
for concave lenses.
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Another method of introducing correcting lenses was a design by Edward
Jackson of Denver, Colo, in 1887.4(p361)He used the length of the ophthalmoscope head and column to incorporate
2 vertically sliding lens racks, one on top of the other, each with 5 lenses
of convex and concave powers that could be used singly or in combination.
In the search for more lenses for refraction, John Couper designed the
first "chain-of-lenses" ophthalmoscope (Figure 12).17 This was a brilliant
engineering feat, with the ophthalmoscope containing no less than 72 lenses,
each mounted in a free-moving brass cell. A cogwheel at the center of the
instrument was used to drive the chain of lenses around a groove in the handle
and head, with each lens coming to rest precisely in the center of the viewing
aperture.
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Figure 12. John Couper's chain-of-lenses
ophthalmoscope with a shaped wooden case.
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The Couper ophthalmoscope was the forerunner of Andrew Stanford Morton's
first ophthalmoscope, in 1884.5(p119) The Morton device was to become the standard chain-of-lenses design
for the next century.
In the attempt to automate the introduction of a wide range of sequential
convex and concave lenses in a conventionally shaped head, perhaps the Roth
and Callan ophthalmoscopes are the most ingenious. Although both ophthalmologists
published details of their instruments in 1864,18-19
Peter Callan's visit to Berlin most likely allowed him to observe August Roth's
design and then imitate it. Only in detail do they differ.
The change in lens power in the Roth ophthalmoscope (Figure 13), diopter by diopter up to 47 diopters, is achieved by
turning a serrated wheel that drives a peg along a wormlike groove, which
rotates the Rekoss disc and linked quadrant. The peg is connected to a pointer
on the outside that indicates the lens power required.
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Figure 13. Roth ophthalmoscope with cover
removed to show the internal groove and peg mechanism for changing lens power.
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The size of correcting lenses in ophthalmoscopes varied enormously,
from 8 mm in the Landolt ophthalmoscope down to 3 mm in others. One early
unidentified ophthalmoscope even had oval-shaped lenses, presumably attempting
to pack the maximum number of lenses within the Rekoss disc yet allowing the
observer plenty of vertical movement (Figure
14).
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Figure 14. Unidentified ophthalmoscope,
circa 1870, with cover (right) removed to expose large oval-shaped lenses
in the Rekoss disc.
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A review of the development of the ophthalmoscope in the lifetime of
Helmholtz would not be complete without mentioning stand-mounted ophthalmoscopes
and the binocular indirect ophthalmoscope.
Many of the best-known ophthalmologists used stand-mounted ophthalmoscopes
like the Follin and Liebreich designs for teaching and studying the fundus
to produce atlases. The first fundus photograph, published by Oswalt Gerloff
of Göttingen, Germany, in 1891,20 had
been preceded by meticulously painted fundi showing a multitude of pathological
conditions that the ophthalmoscope could now reveal.
For today's ophthalmologist, the use of the direct ophthalmoscope is
less frequent. However, the binocular indirect ophthalmoscope, first introduced
in 1861 by the Frenchman Felix Giraud-Teulon,21
is now used as a standard piece of diagnostic equipment. Giraud-Teulon's binocular
indirect ophthalmoscope, which used a solid rhomboid prism to divide the image,
had a fixed interpupillary setting. His instrument was followed in 1862 by
a more sophisticated design by J. Zachariah Laurence and C. Heisch of London.5(p95)
Both instruments were handheld and used for their light source an oil
or gas lamp placed behind and to the side of the patient. These instruments
were difficult to use and quickly went out of fashion. The popularity of binocular
indirect ophthalmoscopy had to wait many years until the introduction of Charles
Schepens' instrument in 1947, which incorporated bright and reliable illumination.22
The invention of the ophthalmoscope by Hermann von Helmholtz was enormously
exciting for the ophthalmologists of the day and fostered respect for and
recognition of ophthalmology as a medical speciality. John Hughlings Jackson,
a renowned English ophthalmologist and neurologist, never tired of impressing
on physicians the value of the routine use of the ophthalmoscope. He stated
that the physician was as much indebted to Helmholtz as the ophthalmologist.23
Eight years after the discovery of the ophthalmoscope, Albrecht von
Graefe, the greatest advocate of this new tool, presented Helmholtz with a
cup at the 1858 Heidelberg Ophthalmological Congress. The cup was inscribed
with words that are as appropriate today as they were then: "To the creator
of a new science, to the benefactor of mankind, in thankful remembrance of
the invention of the ophthalmoscope"24 (Figure 15).
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Figure 15. Photogravure portrait of Hermann
von Helmholtz in 1894, the year of his death.
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AUTHOR INFORMATION
Accepted for publication September 19, 2001.
Corresponding author and reprints: C. Richard Keeler, 1 Brookfield
Park, London NW5 1ES, England (e-mail: rkeeler{at}freenetname.co.uk).
From the Royal College of Ophthalmologists Museum and Library, London,
England.
REFERENCES
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