Methods of Reflecting Light
Helmholtz was the first to observe the human fundus with his Augenspiegel (eye-mirror) in 1851. He had calculated that his line of vision had to be on the same axis as the direction of the illumination.
His solution to this problem was to use laboratory cover plates placed one on top of the other and positioned at an angle such that light was reflected into the patient's eye and the observer could view the fundus through the semi-reflective plates.
He experimented using various numbers of plates, observing that partial polarisation was achieved using three or more.
The painting above is a highly fanciful 1930s retrospective impression of the growth of routine direct examination of the eye in the Victorian period, in this case by a reference to Helmholtz reflecting a spot of light from a lamp behind a filter, using a glass plate, probably a spare microscope slide. The diagram to the right shows how his mirror plates worked.
Helmholtz's 'plates' method of reflection was followed by several ophthalmoscopes using a plano mirror with an aperture scraped from the middle. The first one was Epkens' ophthalmoscope in 1852.
The next year (1853) Adolf Coccius, although using a similar mirror, mounted a biconvex lens on an arm in line with the illumination, thereby focusing the light into the eye in a more concentrated way.
The diagram on the left shows Coccius' ophthalmoscope of 1853 (illuminated by candle light) with its square mirror, aperture and bi-convex lens located at the side.
The first person to introduce a concave mirror with aperture was Theodor Ruete of Leipzig. His claim to fame, in 1852, was to construct the first instrument for the indirect method of ophthalmoscopy. This is illustrated in the diagram below to the right. Note also the glass-protected flame and the elaborately turned mirror stand.
Although Helmholtz had anticipated this method of ocular examination it was Ruete who made it a reality. From then on virtually all ophthalmoscopes could be used in either the direct or indirect mode. To change from one to the other all that was required was to interchange concave mirrors, the one being used in indirect ophthalmoscopy being of lower power. Most instruments included a condensing lens of approximately 13 dioptres.
It was not until 1872 that the dioptre was adopted as the international unit of measurement of the power or focal length of a lens or mirror. Until then, English, Prussian or Paris 'inches' were used.
Direct Ophthalmoscopy = Viewing the retina through the ophthalmoscope, giving a high magnification but a small angle of view.
Indirect Ophthalmoscopy = Viewing the inverted image of the retina which is formed in front of a condensing lens, giving low magnification with a wide angle of view.
One of the early problems when using the direct ophthalmoscope with the light placed to the side was the need to tilt the mirror at an angle to the line of observation for the light to enter the patient's eye. This meant that the observer was viewing the fundus obliquely through the correcting lenses. For low lens powers this was not a problem but in the higher ranges it produced a reduction in vision and a shifting of the image due to the prismatic effect.
In 1875 John Couper of the Royal London Ophthalmic Hospital overcame this problem by dissociating the round disc of lenses from the mirror. By this means the observer was able to look through the centre of the lenses perpendicularly. Couper's design, made by Pickard and Curry of London, went through several models and effectively made redundant the fixed-mirror design.
Edward Loring of the USA improved on Couper's model in 1877 with his vertically tilting mirror. This consisted of a round mirror with the sides cut off, rotatable on its vertical axis. The adjacent drawing shows this very clearly.
There were to be many imitations of this invention over the next twenty years.
The next problem that needed solving was the desirability of having the two concave mirrors mounted on the instrument at the same time instead of having to be taken off and replaced each time.
In 1882 George Lindsay Johnson of London introduced an ophthalmoscope with two mirrors fixed to a plate that could be rotated around a central pivot. In this way each mirror could be positioned quickly behind the sight hole with the smaller 3-inch focal length mirror rotatable around itself for left or right positioning. Later variations of this model featured three or four mirrors, in pairs back-to-back.
Andrew Stanford Morton of London adopted this system and popularised the non-illuminous and self-illuminated ophthalmoscopes bearing his name for over 40 years, but it was not so much the mirror arrangement as the elongated track of lenses for which he is best known.
The colour photograph shows a late 19th century Morton Ophthalmoscope belonging to the museum. It is in its original case along with an auxiliary condensing lens. The practitioner manipulated the lenses via the brass wheel. The handle screwed off for compact storage and included a bonus coloured dot test on the end.
FACT: The BOA Museum has over forty Morton ophthalmoscopes of various versions and dates.
The first ophthalmoscope that Helmholtz constructed in 1851 had no lenses for correcting errors of refraction in the patient and/or the observer. A year later Egbert Rekoss, Helmholtz's machinist at the university, added two rotatable discs, each containing a few lenses. You can see these clearly on the drawing to the left.
Over 150 years later the Rekoss Disc is still used on the majority of hand-held ophthalmoscopes. Although Rekoss had pointed the best way of correcting an out-of-focus image with his wheel of lenses, a number of the early ophthalmoscopes such as those of Coccius (1853), Wilhelm von Zehender (1854), Edward Jaeger (1854) and Richard Liebreich (1855) used separate individual lenses which were inconvenient and took time to change.
The photograph to the right shows a Liebreich ophthalmoscope with lens clip and three auxiliary lenses to use in it, as well as two separate condensing lenses.
Two ophthalmoscopes from a famous private collection:
Jaeger's Ophthalmoscope, as modified by Mauthner in 1865, was supplied with a whole range of additional lenses seen here on the left set into the case lid. This particular example was purchased for the BOA Museum from the important Alfred Schett Collection which went under the hammer in 2002.
In 1869 Edward Loring, in the first of his several ophthalmoscope models over the years, used three interchangeable Rekoss discs, each with eight lenses. One disc contained concave lenses of moderate powers, another had convex lenses of low powers and a third consisted of high powers in both concave and convex form. This increased range of lenses enabled the practitioner to estimate the refractive error of the patient during ophthalmoscopy by introducing the lens which produced the the sharpest image of the fundus. Any refractive error of the observer would be added to or subtracted from the power shown in the disc lens.
In 1873 Hermann Knapp of New York employed a novel way of extending the range without having to interchange discs. He used two Rekoss discs which overlapped one at the bottom and the other at the top. The combination of lenses gave a very wide range of powers rapidly and in small jumps.
[Images to be inserted] Demonstration of Couper chain ophthalmoscope
Head of the Galezowski ophthalmoscope, 1882
In 1882 Xavier Galezowski introduced an even more ingenious arrangement. His ophthalmoscope had one Rekoss disc but there were two circles of lenses within it. The outer of the two concentric rings of 19 lenses was concave whilst the inner ring had 13 convex lenses. To change from one power range to the other the operator merely moved the whole disc up or down within the ophthalmoscope chassis.
Edward Jackson of the USA employed the whole length of the ophthalmoscope head and connecting stem (1887) to mount two sliding racks of lenses, one on top of the other and each with five lenses of concave and convex lenses, which could be moved up or down vertically to align the appropriate power.
Earlier, in 1883, in a radical departure from the Rekoss disc, John Couper had designed the first 'chain of lenses' ophthalmoscope. This brilliantly engineered instrument contained no fewer than than 72 individual lenses, each one mounted in a brass cell and each pushing the next around a track inside the ophthalmoscope when driven by a cogwheel placed at the base of the head.
Our modern demonstration photograph on the right shows how ergonomically designed this 19th century instrument was.
The Couper was the forerunner of Morton's first remarkable ophthalmoscope which was to become the standard 'chain of lenses' design for the next hundred years!
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