It has been observed that De in Eire de nied that there is an alteration in the focus, whether we look at a near or distant object. Ile regarded the whole adjustment as a simple enlargement and diminution of the pupil. At first sight this may appear absurd, but there are facts which give some colour to his theory. That a change in the size of the pupil has a considerable influence upon the distinctness of objects at different distances, is known, and if the eye be turned to a near object, as a book, after it has been gazing at a distant ship, the illumination of both being equal, the pupil is observed to contract. Dr. Mackenzie says, it is an error not unfre quently adopted, that if the rays pass into the eye from a distant object, and those from a near object, have the same divergence, a circumstance which may depend on a mere change in the size of the pupil, they will be collected on the same point of the retina without any change in the refractive media of the eye. That this cannot be the case, is evi dent from the fact that the rays from a dis tant object, and those from a near object, although they may have the same divergence, fall on the cornea at different angles of inci dence, and must necessarily meet the axis of the eye at different points after refraction.
Now the whole effect of the alteration in the size of the pupil is, as explained elsewhere, to increase the distinctness of objects by cutting off those rays which would cause cir cles of dispersion on the retina. On the one hand, an enlargement of the pupil gives this tinctness to distant objects, by allowing a greater quantity of light to enter the eye, and on the other, its contraction assists in render ing near objects distinct by cutting off the lateral rays which are not duly refracted, and would cause confusion of the image on the retina. If, however, a proof were required to show that something more than a mere alter ation in the form of the pupil is necessary to the perfection of vision at all distances, it would be afforded by the change termed pres byopirt. The eye, in what may be called its state of perfect indolent vision, is adapted only to see distant objects, the adjustment to the near focus requiring an effort : the power to make the effort in question is partially or entirely lost by the presbyopic eye, yet the pupil may act as vigorously as ever. Though it is not unusual to find a degree of sluggish ness in the actions of the pupil in elderly persons, yet we have repeatedly seen instances in which the iris acted with great vigour, and where a book was obliged to be held at arm's length for the type to be distinctly seen.
Wagner and Dr. Clay Wallace of New York follow Porterfield in attributing the ad justment to the action of the corpus ciliare ; the latter considering that, " by the graduat ing power of the ciliary processes and ciliary muscles, together with the elasticity of the membranes of the vitreous body, the crystalline may be drawn not only backwards and for wards, but its inclination may be changed so as to throw the image on another part of the re retina :" and the modes operandi he explains by supposing " the outer ciliary muscle to contract the vessels returning from the ciliary pro cesses ; the ciliary processes which are at tached by the filaments of Ammon to the ciliary•zone and crystalline capsule, to become erect and to draw forward the crystalline body ; and the rimer ciliary muscle, aided by the elasticity of the membranes of the vitreous humour, to draw it backwards." The opinion, however, which appears most satisfactory, is that advanced by Mr. Bow man, who has clearly proved the muscular nature of the ciliary body. " It has (says he) the arrangement of a muscle, very much the structure of a muscle, and is largely sup plied with nerves, which are in great part derived from a motor source — the third pair. This muscle arises, or has its most fixed attachment, at the junction of the scle rotica and cornea, as much in front of the lens as is possible, consistently with the preserva tion of the transparency of the cornea. That
it may act more freely, a canal, the circular sinus, is interposed between its origin and the portion of the sclerotica which it lies against. Beyond this point it is hardly at all attached to the sclerotica, over which its fibres may be supposed to move in contraction ; but it covers, and is inserted into, the anterior one eighth of an inch of the choroid membrane, which is in this part tougher and firmer than elsewhere, and united in a very special manner to the lens by the ciliary processes, through the medium of a firm tough membrane, and of a strong elastic fibrous membrane proceed ing•from it to the margin of the lens, and yet not quite to its margin, for an elegant arrange ment. exists, the canal of Petit, by which traction is made, not on the vitreous around the lens, nor on the edge of the lens itself, so much as on its anterior surface. I confess it seems to me very difficult to doubt that this complicated system of parts is intended to advance the lens towards the cornea, so as to bring forward, up to the retina, the focus of a near object, which would otherwise fall be hind the nervous sheet. It is possible also, I think, from the peculiar direction taken by the ciliary muscle, that it may compress the front of the vitreous, and thus help to throw for ward the lens." * An ingenious theory has been suggested by Sturm, and supported by Matteuci, founded on the results of Chossat's measurement of the eye of an ox, to the effect that in place of comparing the optical apparatus of the eye to a system of spherical lenses whose axes are blended, we ought to consider the organ as composed of several refracting media, sepa rated by surfaces which are neither exactly spherical, nor even of revolution or symme trical about a common axis. Reasoning from this, he argues that a peculiar refraction of the rays of light takes place, whereby tne retina is placed in what he terms a focal in terval, which focal interval will change its position according as the external luminous point recedes from, or approaches to, the eye and that the retina will be alwa.is met by the concentrated fasciculus around the axis in the focal interval ; the surface of intersection of this fasciculus and of the retina being very slightly modified, in order that the impression may not be sensibly altered, or the perception rendered indistinct. This theory is, however, decidedly open to objection, and is rendered unnecessary by that of Mr. Bowman.
Magnifying lens. — It has been already stated that the apparent magnitude of an ob ject depends upon that of the angle of vision under which it is seen, and this increases in proportion as the object is brought nearer to the eye ; but the magnitude of the angle of vision being limited, we are obliged to resort to artificial means to enlarge it further than, in its natural condition, is admitted of. The pin-hole aperture affords some assistance, but the convex lens more. The following is the mode in which it acts : Let c D be a convex lens, and A B an object lying within the focal length of the glass, then all the rays passing from a point of the object A 13 will diverge after their passage through the lens, exactly as if they came from the corresponding point of the image ab ; an eye behind the lens would be able to see the ob ject distinctly through the lens if the image ab were at the distance of distinct vision. In this case, however, the object being much nearer the eye, it could not be seen without the lens. The magnifying power of the lens therefore depends essentially on the means it gives us of bringing the object very near the eye, and thus increasing the angle of vision.