History of the Eye
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.
Click photo to enlarge
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 he 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' writings. 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 Young.
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.