in general terms, are the phenomena attending the formation of images in the eye; and they are strictly in accordance with the mode of action of convex lenses, of which the rule is, that an image formed by a convex lens is inverted, and its position relatively to the position of the object and its magnitude, are to that of the object as its distance from the lens is to the distance of the object from the lens. The rays composing a pencil falling upon the cornea, are refracted by the trans parent media of the eye in proportion to the difference between the density of these media, and that of the air, and in proportion to the curves presented by their several surfaces. It is of course the central ray only, or that which passes through the axis of the eye, that is not refracted ; all the other rays un dergo refraction, and are approximated to the central ray. The prime rays have been termed rays of direction, because every prime or axial ray determines the direction of the other rays. As every object emits rays, from every point, in all directions, which rays then proceed in straight lines, it necessarily follows that, un dergoing these refractions, there must be some point in the eye at which the axial rays of the different pencils, proceeding from the ob ject, cross ; and this appears to be very near the centre of the eye, somewhat behind the crystalline. Sir D. Brewster places it in the ge,nnetric centre of the eye-ball, consequently a little within the crystalline lens ; and Volk mann has described the point of intersection as being 3" 97 behind the cornea, 0"/ 43 before the posterior surface of the crystalline, and 6"' 23 in front of the retina. Other and very careful observers, however, place it at a very short distance behind the crystalline lens. It is called the focal centre, and the angle formed by the intersecting axial rays from two points is the visual angle. This focal centre seems, according to the researches of Ruete, to be of importance in another way. In the steady contemplation of objects, we have to bring them into the focal centre of the produced visual axis ; and in the motion by which this is accomplished, it would appear that the eye revolves accurately round a point, which point of revolution is the focal centre of the eye. In vision, the muscles of the two eyes act tinder the influence of the will, with a remark able and admirable sympathy ; and it is on this harmonious consent, as it were, of op posing muscles, that vision in its most perfect form depends.
Vision under water is attended with some curious consequences, the result of what is termed " internal" reflection. An eye placed under perfectly still water, as for instance, the eye of a diver, will see external objects only through a circular aperture (as it were) of 55' 20" in diameter overhead. But all objects down to the horizon will be visible in this space ; those near the horizon being much distorted and contracted in dimensions, espe cially in height. Beyond the limits of this circle will be seen the bottom of the water, and all subaqueous objects reflected and as vividly depicted, as by direct vision ; and in addition, the circular space above mentioned will appear surrounded with a rainbow of faint delicate colours. In the eyes of fishes, the . mours being nearly of the refractive density the medium in which they live, the action bringing the rays to a focus on the retina is most entirely performed by the crystalline ns, which is nearly spherical and of small dius in comparison with the whole diameter of the eye ; there is also a very great in crease of density towards the centre, whereby spherical aberration is obviated, the corneal refraction having little influence.
When speaking of light we have mentioned spherical and chromatic aberrations; and it is necessary that they should be again alluded to in reference to the eye. Spherical aberration is beautifully counteracted by the figure and vary ing density of the crystalline lens ; which, in creasing in refractive power towards the centre, refracts the central rays in each pencil, to the same point as its external rays : but an important agent in obviating this aberration is the iris, which is, as it were, perpetually on the watch to limit the rays entering the eye to those which produce a perfect image, cutting off others which, by their obliquity of incidence, might occasion the imperfection in question. Opticians endeavour to obtain the same effect in their instruments by the employment of an opaque screen or diaphragm ; but no device of human art can equal the ever changing pupil of the living eye.
Sir David Brewster, and some other autho rities, deny that the eye is perfectly free from chromatic aberration: and it is certain that when the pupil is dilated by belladonna, and the lateral rays freely admitted, coloured fringes are perceptible, as we have ourselves experienced ; on the other hand, the forms and relative densities of the humours of the eye closely imitate the achromatic combina tion of lenses, for the two menisci, formed by the aqueous and vitreous humours, having the double convex crystalline lens, of greater density than either, placed between them, fulfil these conditions very happily, and can hardly fail to obviate, in a material degree, chromatic aberration. The coloured fringes we have spoken of, as being produced by the dilatation of the pupil, must not be confounded with the chromatic images, which depend on certain conditions of the retina. The former are always connected with refraction, and they attach particularly to two conditions, namely, the falling of a light close to a shadow, and the projection of the limits of either on the retina, in such a way that all sharpness of outline is lost. The transition from light to shade—the blending of the light and darkness thus produced —gives rise to coloured fringes. This may be shown by taking up a position, at the end of a room, before a brightly illu minated window, and holding up any small object, such as a pencil, before the eye, which must be steadily fixed upon the window sash. Presently prismatic colours appear on either. side of the bars as well as of the pencil. Goethe explained all chromatic phe nomena on the sole ground of modificatitets in light and shade. In the light seen through dull media, according to his theory, yellow is first perceived, then in succession, red, pale blue, blue, violet, black blue, and black ; and he explains dioptrically-formed coloured margins from the subjective side, by a mutual encroachment of the light and dark, the shadow before the light being perceived as blue and violet, and the light before the shadow as red and yellow.