The nature of vision has been the subject
of much speculation since the earliest days of systematized knowledge.
To the natural philosophers of pre-Hippocratic days, vision was the result
of information gathered by antennae-like rays emitted by the eye; these
rays on striking an object were deflected back to the eye, conveying information
of the outer world. Such information would in turn be transmitted along
the hollow tube connecting the eye with the brain. This view, modified
in one form or other, persisted in clinical ophthalmology, though not without
challenge, till the beginning of the 17th century. Whilst the modifications
constitute the history of the development of ocular physiology, the challenges
were merely brilliant asides.
The modifications that this theory underwent are essentially
few. Plato held that in addition to the visual substance that emerges from
the eye to gather information, there was another factor -- rays from the
objects seen, which blend with those of the eye and thus produce vision.
Alexandrain anatomists fixed the seat of vision in the lens, a view that
Galen elaborated when he conceived the retina lining the posterior aspect
of the lens as a mirror in which the object is reflected and thence transmitted
along the optic nerve to the brain.
A radical break from these views were those of atomists
who conceived vision as the result of small particles constantly detaching
tehemeselved from objects and flying in all direction, including the eye.
Aristotle likewise approached the modern conception when he insisted that
things are seen by influences emanating from them, rather than from rays
emerging from the eye. But whilst speculation was rife, actual observation
was not altogether wanting. Amongst the Alexandrians, Ptolemy wrote a treatise
on light; holding with his contemporaries that objects are seen by rays
emerging from the eye, he taught that distance is judged by the length
of the emergent rays, position by their direction, and size by the angle
rays subtend on striking an object. He recognized binocular vision and
diplopia, even to the extent of describing the crossed and uncrossed varieties
of double vision.
The nature of the visual spirits that produce vision was
defined by Galen as pneuma; the pneuma, derived from the brain,
fills the space in front of the iris, dilates the pupil and surrounds the
lens. Short sight resulted from weakness of the visual spirit; though it
passes through the pupil and emerges from the eye it fails to reach an
object in the distance. A later writer (Alexander of Aphrodosias, in the
3rd century), argued that the phosphene seen on sustaining a blow on the
eye was the result of the pneuma becoming inflamed.
The Arabian renaissance brought uneasy stirrings against
the traditional view of vision as the result of energy emanating from the
eye. Ar-Razi compiled a monograph: "On the nature of vision: wherein is
shown that the Eyes are not Radiators of light". But it was not till Alhazen
(Ibn al-Haitam), in the 11th century, that a valid challenge emerged. Basing
himself on the geometry and physics of his day he solved a number of optical
problems, conclusively establishing the view that objects are seen by rays
passing from them towards the eye and not in the reverse direction as was
believed. With Alhazen begins not only modern physiological optics but
modern optics too, and during the Western Middle Ages Robert Grosseteste,
Roger Bacon, John de Peckham and Vitello contributed to the newer optics.
The more substantial optics that thus emerged had little
effect on ophthalmic physiology. The gulf between the academically minded
physicists and the itinerant oculists of the Middle Ages was too vast to
be easily bridged, and even to the physicians the newer optics percolated
but slowly. Maurolycus, Leonardo da Vinci, Plater and Porta haltingly reached
towards the conception of a camera obscura. Porta's statement is
worth quoting, both for its formulation of the newer view on the nature
of vision and for its retention of the fallacious physiology of Galen:
"As objects illuminated by the sun send their light through a narrow hole
in the window-shutter upon a paper placed opposite, exactly so does light,
passing through t he hole of the pupil, produce images of objects looked
at upon the crystalline lens." That the retina and not the lens was the
receiving plate of the eye was held by Plater, but till Kepler were his
views harmonized with those of Porta.
||In Leonardo da Vinci's notebooks
there are many
sketches relating to the questions of optic and
vision. However, his understandings were not
always accurate. The sketches here show the
vertical sections through the scalp and the eye;
the course of the optic nerve is mistakenly shown
to be connected with the anterior ventricle.
Kepler's work is the consummation of that of Alhazen.
With Kepler the eye becomes an optical apparatus obeying the laws indicated
by the Arabian. The camera obscura conception becomes complete- the retina
is the receiving plate, the lens and cornea are refracting media. With
an understanding of the optical properties of the eye came the appreciation
of the significance of myopia and the rational use of glasses.
A number of problems pressed for solution as a result
of Kepler's work. The precise optics involved acceptance of an inverted
image on the retina. That this indeed occurs was shown shortly afterwards
by the Jesuit Father Scheiner in an experiment in which a windown
was made in the posterior pole of animal eye. Scheniner was also responsible
for measuring the indices of refraction of the components of the eye; he
measured the radius of curvature of the cornea by the simple expedient
of placing glass spheres of known curvature alongside the cornea and finding
which sphere gave an image of equal size to the image of a windown seen
on the cornea. But apart from the accurate physical measurements that were
being undertaken, the conception of the eye as an optical instrument precipitated
the problem of accommodation. Obviously if the eye could register impressions
of objects both near and far, it was a dynamic and not a static optical
apparatus. Accommodation was thus recognized as a property of the healthy
eye, and the problem of accommodation formulated by Kepler was to baffle
physiologists for well over two centuries.
Kepler himself held that accommodation was affected by
the ciliary processes either through a change in the form of the eye, the
antero-posterior diameter becoming shorter and the horizontal diameter
wider, thus bringing the retina nearer to the lens, or alternatively that
the lens was moved from its position. Further possibilities were advanced
by other observers. Descartes held that in addition to change in the length
of the eye, which he regarded as due to the action of the extraocular muscles,
there were also changes in the form of the lens, induced by the ciliary
processes. His views as to changes in the form of the lens were supported
by William Briggs. Other (de la Hire, Haller) sought to explain accommodation
on the basis of Scheiner's observation that the pupil contracts during
accommodation; it was held that the elimination of diffusion circles by
contraction of the pupil would account for the clear vision for near objects
in accommodation -- a view supported by the fact that objects are seen
more clearly through a pin-hole. Changes in the curvature of the cornea
were held responsible by Albinus and Ramsden. Supporting the theory that
accommodation is produced by changes in the curvature of the lens, Jurin
advanced the hypothesis that such changes were brought about by displacement
of the Morgagnian fluid of the lens; whilst independent contractility of
the lens was postulated amongst others by Leeuwenhoek and Thomas Young,
who regarded the lens as a muscular structure. Young's laborious investigation
on the structure of the lens failed to demonstrate nerve fibres in
it, though his "full conviction of their existence" was unshaken. in spite
of his faulty anatomy Young nevertheless solved the problem as ot the seat
of accommodation by experiments on his own eyes. He dismissed the cornea
from consideration by finding
that his accommodation was unaffected when he eliminated
the cornea optically. This he did by using a forerunner of the modern contact
glass - a weak objective lens of microscope placed before the eye with
water between the objective and the cornea. Young, who had very prominent
eye, further disproved that the eye elongates during accommodation by clamping
his own eye between two rings, one placed on the anterior surface of the
eye, turned inwards as much as possible, and the other, the ring of a small
key, thrust on the external side between the orbit and the globe till the
phosphened reached the fovea. Thus clamped, the eye could not elongate
during accommodation, and as this was not abolished and as furthermore
the size of the phosphene did not change during accommodation -- as it
would have done if the eye had elongated -- he held that accommodation
is independent of elongation. Young concluded in favour of regarding changes
in the surface of the lens rather that in its position as the responsible
factor. As additional proof that the lens was the seat of accommodation
he pointed to the fact, stressed before him by Porterfield, that in aphakia
accommodation is abolished. The mechanism whereby the lens surfaces changed
could not elucidate. The discovery of the ciliary muscle had to wait another
fifty years, and it was left to Helmholtz by means of his phakoscope to
demonstrate the actual changes in the curvature of the lens and to describe
the nature of accommodation. In doing so Helmholtz rescued Young's
work from under a spate of theories which continued to flourish in spite
of Young's demonstration of their untenability.
Another consummation of the work of Alhazen came with
Donders. The rather florid judgement of Hirschberg is not an exaggeration:
"Donders' work is of that wonderful clearness that is seen in alpine scene
under a marine blue sky; each chapter is like a self-contained valley:
the writing is polished and therefore so penetrating and permanent." Original
observations are not lacking, but these of themselves would not place Donders
in the forefront amongst the immortals. Much the most significant thing
is the critical analysis which pervades his work. Before Donders refractive
errors were classified according to the correcting lens required; myopia
was the condition in which concave lenses were needed, presbyopia in which
convex lenses were required. The puzzling thing about ""presbyopia" was
its occasional occurrence in young people -- "old sight of young people."
Many people before Donders had conceived of hypermetropia; many too had
realized that disturbances in accommodation could result in defective vision.
It was however left to Donders to separate clearly errors of refraction
from those of accommodation. It was he who introduced hypermetropia as
the antithesis of myopia, clearly separating it from presbyopia, thus demolishing
the "old sight of young people."
The concept and the term emmetropia also came from him.
Many years before Donders, Thomas Young had described astigmatism, but
a mass of hazy notions on the subject awaited crystallization in Donders'
Apart from clear classification, the clinical aspect of
refractive errors was well elucidated. Donders introduced the classical
formula for determining the range of accommodation; conceiving presbyopia
as a diminution of the power of accommodation he established the absolute,
binocular and relative range of accommodation, and also showed that the
correction of presbyopia relieves headache. Myopia was critically considered
from analysis of thousands of cases, and the problems it presented as to
heredity, close work, ophthalmoscopic appearances, anatomy, symptoms and
treatment were clearly brought out. The innovations since 1864 when Donders'
classical Anomalies of Refraction and Accommodation was published,
had added or detracted little of material value, though the full benefit
of this work could not be realized till the introduction of the shadow
test by Cuigenet in 1873 and by the use of mydriatics. The new outlook
that Donders contributed to ophthalmology is well illustrated by the fate
of "asthenopia" a term first introduced by Mackenzie in 1830. To Mackenzie,
who regarded the symptoms as due to retinal exhaustion, the condition was
of such serious import that giving up work and long sea-voyages were considered
appropriate treatment. Since Donders, asthenopia has come to stand for
one of the minor ailments.
In the century following Kepler's, attention was being
given to the fundamental physiology of the eye. Mariotte had already discovered
the blind spot in 1668, and Briggs the optic papilla in 1676; Porterfield
in 1759 showed that the blind spot was indeed the entry of the optic nerve.
Porterfield further insisted that the retina and not the crossroad, as
Mariotte believed, was the essential organ of sight. Whilst attention was
being given to after-images and suggestion even advanced that they are
the result of fatigue of the retina, these and allied problems were generally
regarded as beyond explanation. Porterfield well expressed the contemporary
attitude in a passage characteristic of his century, "The connection betwixt
our Ideas and the Motions excited in the Retina, Optic Nerves and Sensorium
is unknown to us, and seems to depend entirely on the Will of God." Binocular
vision, though Briggs had advanced the theory of corresponding points,
was likewise explained in terms of theology; to Porterfield it was a reflex
act of the soul. It was not till the 19th century that progress in these
fields of study became established.
Binocular vision began to become intelligible with the
introduction of the stereoscope by Wheatstone and with the studies of David
Brewster. Studies of the field of vision, though indicated by Thomas Young,
did not begin seriously till taken up by von Graefe, working with sheets
of paper on which he had drawn radiating lines to act as meridians (1855).
The work on colour vision by Helmholtz was likewise, a return to Thomas
Ocular movements too had to wait till the 19th century
for any intensive study. The work of Johan Muller led to the studies of
Listing and to the formulation of Listing's law in 1857.
After Helmholtz had proved that not only the optic disc
but also the optic tracts were insensitive to light, and Muller had shown
that the layer of rods and cones was the recipient element, Weber (1852)
drew attention to the exclusive presence of cones at the macula and the
formulated the theory that the cones alone are the light receiving elements.