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 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 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.