Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/856

Rh 820 EYE moved above the plane of the paper, d will move below, consequently c will move above, and b will appear to sink, (2) The retinal vessels may also be seen by looking at a strong light through a minute aperture, in front of which a rapid to and fro movement is made. Such experiments prove that the sensitive part of the retina is its deepest and most external layer (Jacob s membrane). (4.) Accommodation, or the Mechanism of Adjustment for Different Distances. When a camera is placed in front of an object, it is necessary to focus accurately in order to obtain a clear and distinct image on the sensitive plate. This may be done by moving either the lens or the sensitive plate backwards or forwards so as to have the posterior focal point of the lens corresponding with the sensitive plate. For similar reasons, a mechanism of adjustment, or accom modation for different distances, is necessary in the human eye. In the normal eye, any number of parallel rays, com ing from a great distance, are focussed on the retina. Such an eye is termed emmetropic (fig. 11, A). Another form of eye (B) may be such that parallel rays are brought to a focus in front of the retina, This form of eye is myopic or short-sighted, inasmuch as, for distinct &quot; vision, the object must be brought near the eye, so as to catch the divergent rays, which are then focussed on ths retina. A third form is seen in C, where the focal point, for ordinary dis tances, is behind the re tina, and consequently the object must be held far off, so as to allow only the less divergent or parallel rays to reach the eye. This kind of eye is called hyper- metropic, or far-sighted. For ordinary distances, at which objects must be seen distinctly in every-day life, the fault of the myopic eye may be corrected by the use of concave and of the hyper- metropic by convex glasses. In the first case, the concave glass will remove the posterior focal point a little farther back, and in the second the convex glass will bring it farther forwards ; in both cases, however, the glasses may be so adjusted, both as regards refractive index and radius of curvature, as to bring the rays to a focus on the retina, and consequently secure distinct vision. From any point 65 metres distant, rays may be regarded as almost parallel, and the point will be seen without any effort of accommodation. This point, either at this distance or in infinity, is called the punctum remotum, or the most distant point seen without accommodation. In the myopic eye it is much nearer, and for the hypermetropic there is really no such point, and accommodation is always necessary, If an object were brought too close to the eye for the refrac tive media to focus it on the retina, circles of diffusion would be formed, with the result of causing indistinctness of vision, unless the eye possessed some power of adapting itself to different distances. That the eye has some such power of accommodation is proved by the fact that, if we attempt to look through the meshes of a net at a distant object, we cannot see both the meshes and the object with equal distinctness at the same time. Again, if we look con tinuously at very near objects, the eye speedily becomes fatigued. Beyond a distance of 65 metres, no accommoda tion is necessary ; but within it, the condition of the eye Fig. 11. A, Emmetropic or normal eye; B, Myopic or short-sighted eye; C, Hypermetropic or long-sighted eye. must be adapted to the diminished distance until we reach a point near the eye which may be regarded as the limit of visibility for near objects. This point, called ihepunctum proximum, is usually 12 centimetres (or about 4 inches) from the eye. The range of accommodation is thus from the punctum remotum to the punctum proximum. The mechanism of accommodation has been much dis puted, but there can be no doubt it is chiefly effected by a change in the curvature of the anterior surface of the crys talline lens. If we hold a lighted candle in front and a little to the side of an eye to be examined, three reflections may be seen in the eye, as represented in fig. 12. The first, , is erect, large, and bright, from the 7 anterior surface of the cornea ; the second, b, also erect, but dim, from the anterior surface of the crystalline lens ; and the third, c, inverted, and very dim, from the posterior surface of the lens, or perhaps the concave surface of the vit reous humour to which the convex sur face of the lens is adapted. Suppose the Fl - 12. Reflected three images to be in the position shown in i the figure for distant vision, it will be found that the middle image b moves towards a, on looking at a near object. The change is due to an alteration of the curvature of the lens, as shown in fig. 13. The changes occurring during accom- FIG. 13. Mechanism cf Accommodation. A, The lens during accommodation, showing its anterior surface advanced; I?, The lens as for distant vision ; C, Position of the ciliary muscle. modation are: (1) the curvature of the anterior surface of the crystalline lens increases, and may pass from 10 to 6 mm. ; (2) the pupil contracts ; and (3) the intraocular pressure increases in the posterior part of the eye. An ex planation of the increased curvature of the anterior surface of the lens during accommodation has been thus given by Helmholtz. In the normal condition, that is, for the emmetropic eye, the crystalline lens is flattened anteriorly by the pressure of the anterior layer of the capsule ; during accommodation, the radiating fibres of the ciliary muscle pull the ciliary processes forward, thus relieving the tension of the anterior layer of the capsule, and the lens at once bulges forward by its elasticity. The exact mechanism of the ciliary muscle is still not clearly understood. Helmholtz has succeeded in measuring with great accu racy the sizes of these reflected images by means of an in strument termed an ophthalmometer, the construction of which is based on the following optical principles. When a luminous ray traverses a plate of glass having parallel sides, if it fall perpendicular to the plane of the plate, it will pass through without deviation; but if it fall obliquely on the plate (as shown in the left hand diagram in fig. 14) it under goes a lateral deviation, but in a direction parallel to that of the incident ray, so that to an eye placed behind the glass plate, at the lower A, the luminous point, upper A, would be in the direction of the prolonged emergent ray, and thus there would be an apparent lateral displacement of the point, the amount of which would increase with the