The Ophthalmoscope
The structure of the interior of the eye had been known with reasonable
accuracy from the time of Galen[C E 131-201] whose observations were based
on dissection of the eyes of animals. Further knowledge had to wait until
the time of Leonardo da Vinci [1452-i519] who dissected the eye following
hardening by boiling it in white of egg.
From the time of Pliny the Elder [C E 23-79, it was observed that the
eyes of various animals would shine in the dark when exposed to light.
Much later is was realised that this was due to reflection from the tapetum.
Both Purkinje in 1824 and William Cumming in 1846 described the red reflex
seen in the pupil under certain lighting conditions. It was obvious that
the interior of the eye could be illuminated but the problem was to view
it.
Diagram 1: Johannes Evangelista Purkinje
The first person to see the interior of the living eye was Jean Mery in
Paris in 1704 who was conducting an experiment to observe the dilating
of a cat's pupil when it became anoxic following being held under water.
When looking through the water he was able to see the retina due to the
elimination of the refractive surface of the cornea.
Charles Babbage, a Cambridge mathematician, devised an instrument in 1847
to view the retina. He showed this to ophthalmic surgeon Thomas Wharton
Jones who said that it seemed to serve no useful purpose so it was discarded.
Diagram 2: Hermann Von Helmholtz in 1848. Aged 26
In 1850, German physiologist Hermann Helmholtz announced that he had
solved the problem of viewing the retina. He realised that rays of light
reflected back through the pupil followed the same path as they did when
entering it. He therefore glued three glass cover slips together and used
them as a mirror to direct light from a lateral source into the pupil.
He then placed his own eye in such a position as to look along this path
of light. The rays of light passing back from the subject's were focused
onto the observer's retina following passage through a concave lens. [see
illustration].
Various modifications soon followed, all using reflected light from oil
or gas lamps. Mirrors with a central perforation quickly replaced the
reflecting glass slides.
Unless both the observer and the subject were emmetropic, lenses had to
be placed in front of the observer's eye to overcome this problem. These
were inserted in hinged frames or placed around the margin of a rotating
Rekoss disc [see illustration]. Some later models used a continuous chain
of lenses within the body of the instrument [see illustration].Modifications
were made so rapidly that there were 41 models by 1862.
All these instruments, except that of Helmholtz, could be used by either
the direct or indirect method, many being supplied with a condensing lens
for that purpose.
Some instruments had more than one mirror, a double sided large one with a
plane and a concave surface for indirect ophthalmoscopy, and a smaller
angled double sided one for direct. [see illustration]
Early ophthalmoscopes were difficult to use, mainly due to difficulty
using the mirror and the low light intensity from the lamps available.
Even 20 years after Helmholtz's, discovery, very few doctors outside Germany
were using the instrument.
The development of electric light in 1878 heralded the development of
electric ophthalmoscopes in New York in 1885 and in London in 1886. However,
it was not until 1910 that they became practical with the development
of small bulbs and dry cell batteries. Because they became easier to use, direct ophthalmoscopes continued to
develop and were the main means of ophthalmoscopy until the 1960's. Large
ophthalmoscopes with high light output such as Keeler's Pantoscope were
produced, which were designed for both direct and indirect ophthalmoscopy,
but were mainly used by the direct method as most operators found this
easier.
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