In the article Optics, n° 154. (Encycl.), it is said, that as we have a power of contracting or relaxing the ligamenta ciliaris, and thereby altering the form of the crystalline humour of the eye, we hence see objects distinctly at different distances. It appears, however, from some experiments made by Mr Everard Home and Mr Ramden, in the year 1793, that this power of contracting and relaxing the ligaments ciliaris is not alone sufficient to account for the phenomenon. Conversing with Mr Home on the different ideas of the crystalline humour, Mr Ramden said, that as that humour "consists of a substance of different densities, the central parts being the most compact, and from thence diminishing in density gradually in every direction, approaching the vitreous humour on one side, and the aqueous humour on the other; its refractive power becomes nearly the same with that of the two contiguous substances." That some philosophers have stated the use of the crystalline humour to be, for accommodating commodating the eye to see objects at different distances; but the firmness of the central part, and the very small difference between its refractive power near the circumference and that of the vitreous or the aqueous humour, seemed to render it unfit for that purpose; its principal use rather appearing to be for correcting the aberration arising from the spherical figure of the cornea, where the principal part of the refraction takes place, producing the same effect that, in an achromatic object glass, we obtain in a less perfect manner by proportioning the radii of curvature of the different lenses.

In the eye the correction seems perfect, which in the object glass can only be an approximation; the contrary aberrations of the lenses not having the same ratio: so that, if this aberration be perfectly corrected, at any given distance from the centre, in every other it must be in some degree imperfect.

"Pursuing the same comparison: In the achromatic object glass we may conceive how much an object must appear fainter from the great quantity of light lost by reflection at the surfaces of the different lenses, there being as many primary reflections as there are surfaces; and it would be fortunate if this reflected light was totally lost. Part of it is again reflected towards the eye by the interior surfaces of the lenses; which, by diluting the image formed in the focus of the object glass, makes that image appear far less bright than it would otherwise have done, producing that milky appearance so often complained of in viewing lucid objects through this sort of telescope.

"In the eye, the same properties that obviate this defect, serve also to correct the errors from the spherical figure, by a regular diminution of density, from the centre of the crystalline outward. Every appearance shows the crystalline to consist of laminae of different densities; and if we examine the junction of different media, having a very small difference of refraction, we shall find that we may have a sensible refraction without reflection. Now, if the difference between the contiguous media in the eye, or the laminae in the crystalline, be very small, we shall have refraction without having reflection; and this appears to be the state of the eye; for although we have two surfaces of the aqueous, two of the crystalline, and two of the vitreous humour, yet we have only one reflected image; and that being from the anterior surface of the cornea, there can be no surface to reflect it back, and dilute an image on the retina.

"This hypothesis may be put to the test whenever accident shall furnish us with a subject having the crystalline extracted from one eye, the other remaining perfect in its natural state; at the same time we may ascertain whether or not the crystalline is that part of the organ which serves for viewing objects at different distances distinctly. Seeing no reflection at the surface of the crystalline, might lead some persons to infer that its refractive power is very inconsiderable; but many circumstances show the contrary: yet what it really is may be readily ascertained by having the focal length and distance of a lens from the operated eye, that enables it to see objects the most distinctly; also the focal length of a lens, and its distance from the perfect eye, that enables it to see objects at the same distance as the imperfect eye: these data will be sufficient whereby to calculate the refractive power of the crystalline with considerable precision.

"Again, having the spherical aberration of the different humours of the eye, and having ascertained the refractive power of the crystalline, we have data from whence to determine the proportional increase of its density as it approaches the central part, on a supposition that this property corrects the aberration.

"An opportunity presented itself for bringing the observations of Mr Ramsden, respecting the use of the crystalline lens, to the proof. A young man came into St George's Hospital with a cataract in the right eye. The crystalline lens was readily extracted, and the union of the wound in the cornea took place unattended by inflammation; so that the eye suffered the smallest degree of injury that can attend so severe an operation. The man himself was in health, 21 years of age, intelligent; and his left eye perfect; the other had been an uncommonly short time in a diseased state, and 27 days after the operation appeared to be free from every other defect but the loss of the crystalline lens.

"A number of experiments were made on the imperfect eye, assisted by a lens, and compared with the perfect eye. The aim of these trials, which were judiciously varied, was to ascertain whether the eye which had been deprived of the crystalline lens was capable of adjusting itself to distinct vision at different distances. Among other results, the perfect eye, with a glass of 6½ inches focus, had distinct vision at 3 inches; the near limit was 1½ inch, the distant limit less than 7 inches. The imperfect eye, with a glass 2½ inches focus, with an aperture 3/8ths of an inch, had distinct vision at 2½ inches, the near limit 1½ inch, and the distant limit 7 inches. The accuracy with which the eye was brought to the same point, on repeating the experiment, proved it to be uncommonly correct; and as he did not himself see the scale used for measurement, there could be no source of fallacy. From the result of this experiment, it appears that the range of adjustment of the imperfect eye, when the two eyes were made to see at nearly the same focal distance, exceeded that of the perfect eye. Mr Ramsden suggested a reason why the point of distinct vision of the imperfect eye might appear to the man himself nearer than it was in reality; namely, that from the imperfection of this organ he might find it easier to read the letters when they subtended a greater angle than at its real point of distinct vision. The experiments, however, appear to show that the internal power of the eye, by which it is adjusted to see at different distances, does not reside in the crystalline lens, at least not altogether; and that if any agency in this respect can be proved to reside in the crystalline, the other powers, whatever they may be, are capable of exertion beyond their usual limits, so as to perform its office in this respect.

"From these considerations, and in consequence of other reflections tending to show that an elongation of the optical axis is not probably the means of adjustment, these philosophers directed their enquiries to ascertain how far the curvature of the cornea might be subject to change. They found by trial that this part of the organ possesses a degree of elasticity which is very considerable, both for its perfection and its range; and by anatomical..." anatomical dissection it was found that the four straight muscles of the eye do in effect terminate in the cornea at their tendinous extremities; that the whole external lamina of the cornea could by gentle force be separated, by means of these muscles, from the eye; so that the tendons seem lost in the cornea, and this last has the appearance of a central tendon. It was also seen that the central part of the cornea is the thickest and the most elastic.

"These were considerable advances towards establishing the hypothesis of adjustment by the external curve of the eye. It remained to be shown, by experiments on the living subject, that this curve does really vary in the due direction, when the mind perceives the distinct visible sensation of objects at different distances. For this purpose Mr Ramsden provided an apparatus, consisting of a thick board steadily fixed, in which was a square hole large enough to admit a person's face; the forehead and chin resting against the upper and lower bars, and the cheek against either of the sides; so that when the face was protruded, the head was steadily fixed by resting on three sides; and in this position the left eye projected beyond the outer surface of the board. A microscope, properly mounted, so as with ease to be set in every requisite position, was applied to view the cornea with a magnifying power of thirty times. In this situation, the person whose eye was the object of experiment was directed to look at the corner of a chimney, at the distance of 235 yards, through a small hole in a brass plate, fixed for that purpose; and afterwards to look at the edge of the hole itself, which was only six inches distant. After some management and caution, which the delicate nature of these experiments requires, the motion of the cornea, which was immediately perceptible, became very distinct and certain. The circular motion of its surface remained in a line with the wire in the field of the microscope, when the eye was adjusted to the distant object, but projected considerably beyond it when adapted to the near one. When the distant object was only 90 feet from the observer, and the near object six inches, the difference in the prominence of the cornea was eliminated at 1800th of an inch. These experiments were repeated and varied at different times and on different subjects. The observer at the microscope found no difficulty in determining, from the appearance of the cornea, whether the eye was fixed on the remote or the near object.

From these different experiments Mr Home considers the following facts to have been ascertained:

1. That the eye has a power of adjusting itself to different distances when deprived of the crystalline lens; and therefore the fibrous and laminated structure of that lens is not intended to alter its form, but to prevent reflections in the passage of the rays through the surfaces of media of different densities, and to correct spherical aberration.

2. That the cornea is made up of laminae; that it is elastic, and when stretched is capable of being elongated 1/11th part of its diameter, contracting to its former length immediately upon being left to itself.

3. That the tendons of the four straight muscles of the eye are continued on to the edge of the cornea, and terminate, or are inserted, in its external lamina; their action will therefore extend to the edge of the cornea.

4. That in changing the focus of the eye from seeing with parallel rays to a near distance, there is a visible alteration produced in the figure of the cornea, rendering it more convex; and when the eye is again adapted to parallel rays, the alteration by which the cornea is brought back to its former state is equally visible."

Mr Home made many other experiments with a view to throw light upon this curious subject; and the result of the whole appears to be, that the adjustment of the eye is produced by three different changes in that organ; an increase of curvature in the cornea, an elongation of the axis of vision, and a motion of the crystalline lens. These changes, in a great measure, depend upon the contraction of the four straight muscles of the eye. Mr Ramsden, from computations grounded on the principles of optics and general state of the facts, estimates that the increase of curvature of the cornea may be capable of producing one-third of the effect, and that the change of place of the lens, and elongation of the axis of vision, sufficiently account for the other two-thirds of the quantity of adjustment necessary to make up the whole.