Why the pupil is black was a problem that attracted
the attention of Roman writers, but the explanations they gave are merely
of historical interest. It was held that the moisture in the eye was black;
it was also suggested that the blackness resulted from the eye being a
sort of deep trough. Yet the Ancients were also acquainted with the fact
that some animal eyes are lustrous. Pliny observed that the eyes of nocturnal
animals, such as cats, are brilliant in the darkness. The explanation had
to wait for many centuries. Mariotte made some approach to it when he noted
that the reason a dog's eye is luminous was that its "choroid" is white,
and that hence the image of a light is painted on it clearly, whereas in
man and in animals with black "choroid" no such clear image could be formed.
This is a dim realization of the existence of the tapetum. Bidloo in the
17th century appreciated that no animal eye radiates light that it had
not received, but it was not till 1810 that the simple observation that
animal luminosity disappears in complete darkness was established by Prevost.
This laid forever such views as those that regarded animal luminosity as
a sort of phosphorescence; or that the radiation by night came from light
absorbed during the day, or yet again that luminosity was the result of
some such internal activity as is seen in the firefly. "Naked electricity"
was yet another explanation that had passed muster. All these views had
been invoked to explain the supposition that animals with lustrous eyes
could see in the dark.
Further advance towards a clearer understanding was supplied
by the work of Rudolphi in 1821. He showed that the luminosity of an animal
eye depended largely upon the direction of the ingoing rays. That furthermore
the problem was a purely physical one he showed by the observation that
the eye of a decapitated cat was just as effective for the production of
luminosity as that of the living animal. A few years later Esser went further
still by showing that the decapitated cat was really the better as the
pupil was widely dilated.
That, at least partially, an optical problem underlay
this animal luminosity was indeed also realized, more that a century earlier,
when Mery in 1703 found that the luminosity of the cat's eye could easily
be viewed when the animal was held under water. He appreciated that it
was more than mere dilatation of the pupil consequent on suspended animation
that was responsible for this phenomenon, and his explanation was that
the water filled in unevenness of the cornea. The correct explanation was
advanced by de la Hire six years later, when he argued that the cat's fundus
was seen owing to abolition of corneal refraction under water; that consequently
the rays emerged divergent, and some of them were thus caught by the observer's
These considerations all seemed to have no practical significance.
Even when luminosity in human eyes was observed the problem still remained
an academic exercise. Duddell, in 1735, had noted the spontaneous luminosity
of the eye of the human albino, as Woolhouse before him had observed it
in white rats. Later in the century Fermin had noted luminosity of the
eye of an Ethiopian albino (and incidentally held that this patient could
thus see at night, because his eyes were like those of night animals).
Further interest in spontaneous luminosity of man was aroused by Richter's
observation (1790) that in one form of blindness luminosity was present.
This led Beer to introduce the term amaurotic pseudo-glioma. Spontaneous
luminosity was also noted in aniridia in 1829 by Beer.
No attempt to explain this spontaneous luminosity in man
was made, though a close approach had been made to the explanation in the
case of animals. The observation of Purkinje in 1823 that under certain
conditions of illumination human eyes could be made luminous, passed unnoted.
It had to be rediscovered independently by Cumming in 1846 and by Brucke
in 1847. It was finally realized that the observer had to stand in the
path of the emerging rays. Brucke indeed came near to inventing the ophthalmoscope
when he looked through a tube placed in the flame of a candle illuminating
the eye, and thus caught some of the emergent rays.
A conscious attempt to see the fundus was made by Kussmaul
at about the same time (1845). On the basis of de la Hire's explanation
of Mery's observation of the fundus of a cat submerged under water, he
applied to the eye a plano-concave lens of the same power as the
cornea, hoping to be thus enabled to see the optic nerve in the living
human eye -- a procedure that " should be of great value in the diagnosis
of certain eye diseases". He failed, for he did not realize the necessity
for illuminating the eye. Babbage, of calculating machines fame, is another
precursor; but whether he acted just as consciously as Kussmaul and what
exactly he invented, is not definitely known. There is no documentary evidence
as to what he made and what he showed to Wharton Jones in 1847, except
the latter's account seven years later.
By this time the optical problem underlying luminosity
of animal eyes and of the human eye under certain conditions had nearly
reaches its solution. Indeed, the fact that the eye was not luminous under
normal conditions because it forms an optical apparatus which returns entering
rays to a focus at the source of illumination, had been indicated by the
rather crude experiments of Kussmaul. THough he had failed to view the
fundus in the living eye by neutralizing the refraction of the cornea,
he showed that by further deranging the optical structure of the eye through
removal of both the cornea and lens the fundus could be seen, and that
it could likewise be seen if some vitreous was extracted and the retina
The crowning achievement came when Helmholtz announced the invention
of an "eye-mirror" in December, 1850. His ophthalmoscope consisted not
of a mirror but of plates of glass, four plates being used to increase
the number of rays reflected into the eye. The illumination was of necessity
poor. Modifications followed each other in rapid succession, the silvered
mirror with a central hole arriving within a year. Two great improvements
were likewise introduced at an early stage. Helmholtz's original ophthalmoscope
was mounted with a holder for one lens, and lenses had to be changed constantly
for eyes of different refraction. Rekoss, a technician, introduced a revolving
disc carrying a series of lenses, whilst Ruete in 1852 introduced the indirect
method of ophthalmoscopy. Thereafter an endless series of modification
and improvements followed. The refracting ophthalmoscope was introduced
at about 1870, whilst tentative electric ophthalmoscopes were brought out
about fifteen years later, one of the earliest being that of Juler in 1886.
Search for the ideal source of illumination led to attempts with oil, petrol,
gas, daylight and almost every conceivable monochromatic flame.
Early indirect ophthalmoscope. Note the inverted image
illuminated with the light from a lamp placed on the
The introduction of the ophthalmoscope in clinical ophthalmology was
facilitated through it is brilliant application by von Graefe and his colleagues.
Enthusiastically received by them it did not fare so well elsewhere. It
was argued that it is dangerous for a diseased eye to be submitted to the
strain of all this illumination. Some, more patronizingly, held that it
might be quite a useful instrument for such oculists as have poor sight.
Dixon of London, 1853, expressed the fear that its use might lead to amaurosis.
In France, qualified support was given to the ophthalmoscope -- as indeed
was the case with earlier models having a concave lens only. It was Anagnostakis
who, in 1854, popularized the instrument in France by a series of excellent
observations. In England, pioneer work was done by Spencer Watson, and
ardent support came from Bowman, though as late as 1855 the Lancet
could still speak sceptically of its value. In other countries, Holland
excepted, it penetrated even more slowly. Yet by the time the First International
Ophthalmological Conference was held in 1857 the ophthalmoscope had come
to be sufficiently significant to claim the first discussion.
Ophthalmoscopes from the 19th century.
Within a decade the ophthalmoscope had revolutionized ophthalmology.
For one thing, it forced attention to the refractive state of the eye,
supplying at the same time objective means of determining it. In no small
measure the work of Donders is the result of the introduction of the ophthalmoscope.
But even more far-reaching was the demolition of the age-long puzzle of
amaurosis. At one stroke endless guesses, speculations, theories and discussions
became meaningless. A new conception of glaucoma emerged early, even if
at first it was held that in glaucoma no changes were present in the fundus,
a view that was replaced by the belief that swelling of the disc was present,
But by 1855 von Graefe, who with others had fallen into the earlier error
as to swelling, demonstrated excavation and retinal pulsation - and iridectomy
as a method of treatment of the hitherto hopeless and badly understood
disease followed rapidly. A new chapter -- medical ophthalmology -- was
opened by von Graefe in 1855 and Heymann in 1856 by the description of
renal retinitis, whilst in 1860 von Graefe presented a boon to neurology
by his observation of bilateral papilloedema. Coccius in 1853 described
detachment of the retina and indicated retinitis pigmentosa. Thrombosis
of the central vein was recognized by Liebriech in 1855, whilst von Graefe
recognized embolism of the central artery in 1860.
Early illustrations of the fundi from the 19th century
were first invented. The picture on the left shows
advanced glaucoma and on
the right retinitis pigmentosa.
Amaurosis, the condition which had been defined as one in which the
patient saw nothing and the oculist also saw nothing , had ceased to exist.