Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/852

Rh 816 E Y E E Y E found, combined with angularity of drapery, and some awkwardness of attitude, in a full length portrait couple at the National Gallery (1434), in which a rare insight into the detail of animal nature is revealed in a study of a terrier dog. A Madonna with Saints, at Dresden, equally soft and minute, charms us by the mastery with which an archi tectural background is put in. The bold and energetic striving of earlier days, the strong bright tone, are not equalled by the soft blending and tender tints of the later ones. Sometimes a crude ruddiness in flesh strikes us as a growing defect, an instance of which is the picture in the museum of Bruges, in which Canon Van der Paelen is represented kneeling before the Virgin under the protection of St George (1434). From first to last Van Eyck retains his ability in portrait. Fine specimens are the two male likenesses in the gallery of Vienna (1436), and a female, the master s wife, in the gallery of Bruges (1439). His death in 1440-41 at Bruges is authentically recorded. He was buried in St Donat. Like many great artists he formed but few pupils. Hubert s disciple, Jodocus of Ghent, hardly does honour to his master s teaching, and only acquires importance after he has thrown off some of the peculiarities of Flemish teaching. Petrus Cristus, who was taught by John, remains immeasurably behind him in every thing that relates to art. But if the personal influence of the Van Eycks was small, that of their works was immense, and it is not too much to say that their example, taken in conjunction with that of Van der Weyden, determined the current and practice of painting throughout the whole of Europe north of the Alps for nearly a century, (j. A. c.) EYE THE sense of vision is excited by the influence of light on the retina, the special terminal organ connected with the optic nerve. By excitation of the retina, a change is induced in the optic nerve fibres, and is conveyed by these to the brain, the result being a luminous perception, or what we call a sensation of light or colour. If light were to act uniformly over the retina, there would be no image of the source of the light formed on that structure, and consequently there would be only a general conscious ness of light, without reference to any particular object. One of the first conditions, therefore, of vision for useful purposes is the formation of an image on the retina. To effect this, just as in a photographic camera, refractive structures must be placed in front of the retina which will so bend luminous rays as to bring them to a focus on the retina, and thus produce an image. Throughout the animal kingdom, various arrangements are found for this purpose ; but they may be all referred to three types, namely (1) eye-specks or eye-dots, met with in Medusa 3, Anndidse, &c.; (2) the compound eye, as found in insects and crustaceans; and (3) the simple eye, common to all vertebrates. The eye- specks may be regarded simply as expansions of optic nerve filaments, covered by a transparent membrane, but having no refractive media, so that the creature would have the consciousness of light only, or a simple luminous impression, by which it might distinguish light from darkness. The compound eye (an account of which, as met with in the com mon lobster, will be found under CRUSTACEA, vol. vi. p. 637) consists essentially of a series of transparent cone-like bodies, arranged in a radiate manner against the inner sur face of the cornea, with which their bases are united, while their apices are connected with the ends of the optic filaments. As each cone is separated from its neighbours, it admits only a ray of light parallel with its axis, and its apex represents only a portion of the image, which must be made up, like a mosaic-work, of as many parts as there are cones in the eye. When the cones are of considerable length, it is evident, from their form and direction, their apices being directed inwards, that the oblique rays emanat ing from a luminous surface will be cut off, and that only those rays proceeding along the axis of the cone will pro duce an effect. Thus distinctness or sharpness of definition will be secured. The size of the visual field will depend on the form of the eye, the outermost cones marking its limits. Consequently the size of the visual field will depend on the size of the segment of the sphere forming its surface. The eyes of many insects have a field of about half a sphere, so that the creature will see objects before and behind it as well as those at the side. On the other hand, in many the eyes have scarcely any convexity, so that they must have a narrow field of vision. A description of the simple eye will be found in the article ANATOMY, vol. i. p. 885 sq. Optically, it consists of a series of refractive media placed in front of the retina by which rays emanating from an external object are brought to a focus on that structure. In this article, we shall con sider (1) the physical causes of vision; (2) the optical arrangements of the eye; (3) the specific influence of light on the retina; (4) sensations of colour; (5) the movements of the eyes in vision; and (6) the psychical relations of luminous impressions. 1. PHYSICAL CAUSES OF VISION. A luminous sensation may be excited by various modes of irritation of the retina or of the optic nerve. Pressure, cutting, or electrical shocks may act as stimuli, but the normal excitation is the influence of light on the retina. From a physical point of view, light is a mode of movement occurring in a medium, termed the ether, which pervades all space ; but the physiologist studies the operation of these movements on the sentient organism as resulting in consciousness of the particular kind which we term a luminous impression. Outside of the body, such move ments have been studied with great accuracy ; but the physiological effects depend upon such complex conditions as to make it impossible to state them in the same precise way. Thus, when we look at the spectrum, we are conscious of the sensations of red and violet, referable to its two extremities : the physicist states that red is pro duced by 392 billions of impulses on the retina per second, and that violet corresponds to 757 billions per second; but he has arrived at this information by inductive reasoning from many facts which have not at present any physiological explanation. We cannot at present trace any connexion, as cause and effect, between 392 billions of impulses on the retina per second and a sensation of red. Below the red and above the violet ends of the spectrum there are vibrations which do not excite luminous sensations. In the first case, below the red, the effect as a sensation is heat; and above the violet the result is that of chemical activity. Thus the method of dispersion of light, as is followed in passing a ray through a prism, enables us to recognize these general facts: (1) rays below the red excite thermal impressions ; (2) from the lower red up to the middle of the violet, the thermal rays become gradually weaker until they have no effect; (3) from the lower red to the extreme violet, they cause luminous impressions, which reach their greatest intensity in the yellow ; and (4) from about the end of the yellow to far beyond the extreme violet, the rays have gradually a less and less luminous effect, but they have the power of exciting such chemical changes as are