Page:The New International Encyclopædia 1st ed. v. 07.djvu/446

EYE. . U EYE. The adaptation of the eye to distinct, vision at every distance beyond that of a few inches is ex- tremely remarkable. The process to which the structures of the eye lend themselves in accom- plishing this is known as i dation. The essential factor in the process is the contraction of the ciliary muscle, which, as previously noted, allows the suspensory ligament of the lens to relax, with a resultant bulge of its anterior sur- face and a decrease in its focal length. Accord- ing to Helmholtz, the radius of curvature of the anterior surface of the lens diminishes on turning the eye to a near object from ten to six milli- meters (from about 0.4 to 0.24 of an inch) . while the most projecting point of the same surface is brought forward about 0.2 of an inch. Accord- ing to the observations of Hueck, the focal dis- tance may be changed about three times in a sec- ond. The accommodation from a near to a dis- tant object is effected much more rapidly than the converse process. There are two well-known forms of defective vision in which this power of adaptation is very much limited — viz. short-sightedness, or myopia, and long-sightedness, or hypermetropia. The lim- itation, however, is not due to a defect in the muscular apparatus to which we have referred, but to an abnormality either in the curves of the refracting media or to congenital or acquired changes in the anteroposterior diameter of the ball. In short-sightedness from too great a re- fractive power from either cause, the rays from - at the ordinary range of distinct vision are brought too soon to a focus, so as to cross one another, and begin to diverge before they fall on the retina, the eye in this case being able to bring to the proper focus on the retina only those rays which were previously diverging at a large angle from a very near object. The correction for this deficiency is accomplished by interpos- ing between the eye and indistinctly seen objects a concaw lens, with a curvature just sufficient to - of external objects at the ordi- nary distance of distinct vision backward upon ina. In far-sightedness, on the othi there is an abnormal diminution of the refractive power from too flat a cornea, a deficient aqueous "i :i flattening of the lens, so that the fo- cus is behind the retina. This defect is corrected by convex lenses, which increase the convergence of the rays of light. Presbyopia, as its name in- dicates, usually conies on at a comparatively ad- vanced period of life, and is due to senile changes affecting the elasticity of the lens and its attach- ments. We have already noticed the most essential use of the iris — viz. its power, under the influence of light upon the retina, of modifying the si/.,, of the pupil so a^ I unit of light entering the eye. I'ut this is not its only use, being to pi i i en! t he pa cin imferen-tia) part of I hi ii I thrj to oti iate t he ind) I i i irould arise i ical aberra- tal refraction of the rays passing rid near the margin of the i manner as 1 he diaphragn itii in Bui 'Mil ion- i ' this spherical ( illted. which so well illu^l rate tt Of the eye i ice them. They arc described by W'lin r' lows: (1) "The surfaces of the dioptric parts of the eye are not spherical, but those of the cornea and posterior surface of the lens are hyperbolical, and that of the anterior surface of the lens ellip- tical — configurations found by theory fitted to prevent spherical aberration. This discovery was made at a time when it was not known but that the dioptric parts of the eye had spherical sur- faces. (2) "The density of the lens diminishing (as we have already shown) from the centre to its periphery, the circumferential rays are less refracted than they would have been by a homo- geneous lens with similar surfaces. This ele- gantly simple contrivance has been hitherto in- imitable by human art." Chromatic aberration, which is caused by the unequal refrangibility of the primitive rays of which white light is composed, when trans- mitted through an ordinary lens, whereby colored fringes are produced, is practically corrected in the eye, although it is doubtful whether it is entirely absent. The provision, however, on which the achromatism depends has not. been deter- mined with certainty, probably because we do not yet know the relative refractive and dispersive powers of the cornea and humors of the eye. Sir David Brewster denies that the chromatic aber- ration receives any correction in the eye, and maintains that it is imperceptible only in conse- quence of its being extremely slight. See also Vision. EYE, Comparative Anatomy of the. How far down in the scale it is necessary to go to fine animals which have no eyes depends upon wheth- er we mean by 'eye' an organ capable of form- ing optical images, or simply an organ capable of responding to the stimulus of light. If the latter is really an eye, then we are justified in saying that all the large groups of animals higher than sponges have eyes, though many families and even some orders may lack them. Among ccelen- terates, simple light-detecting organs, known as pigment spots, or 'pigment eyes,' and consist- ing of groups of pigment-cells associated with sensory cells, occur in many medusa; and in ctenophores. In some cases the cuticle over these spots is specially thickened to form a sort of a lens. Similar pigment eyes occur among the fiat- worms (Platoda), and in some cases they are somewhat more complicated by the additions of so-called retinal cells with rod-like processes. It is very doubtful, however, whether these i are really anything more than light-detecting organs. Pigment eyes very similar to these, though often somewhat more complicated, occur in many worms, crustaceans, insects, and niol- lusks, and in a few echinoderms. The eyes of tunicates and of Amphioxus are not of any higher degree of organization. In crustaceans the pig- ment eye is known as the 'unpaired' eye. and is apparently made up of three simple eyes h ether. In insects the pigment eyes arc called 'ocelli.' and these also occur in spiders and scor Pigment eye- arc of service to their p h na bling i hem to disl inguish bet ween light and shade, and in detecting differenl dej light. Thus shadows east by an approaching enemy would be noticed, and. in aquatic animals. approach to the surface would be quickly indi- cated. Turning now to tli ins which unques tionably form some sort of an image, we find