818 EYE are (i) it i s reversed ; (2) it is sharp and well defined if it be accurately focussed cm the retina ; and (3) its size depends on the visual angle. If we look at a distant object, say a star, the rays reaching the eye are parallel, and in passing through the refractive media, they are focussed at the posterior focal point, that is, on the retina. A line from the luminous point on the retina passing through the nodal point is called the line of direction. If the luminous object be not nearer than, say, 60 yards, the image is still brought to a focus on the retina without any effort on the part of the eye. Within this distance, sup posing the condition of the eye to be the same as in look ing at a star, the image would be formed somewhat behind the posterior focal point, and the effect would be an indis tinct impression on the retina. To obviate this, for near distances, accommodation, so as to adapt the eye, is effected by a mechanism to be afterwards described. When rays, reflected from an object or coming from a luminous point, are not brought to an accurate focus on the retina, the image is not distinct in consequence of the formation of what are called circles of diffusion, the pro duction of which will be rendered evident by fig. 4. From FIG. 4. Formation of Circles of Diffusion. the point A luminous rays enter the eye in the form of a cone, the kind of which will depend on the pupil. Thus it may be circular, or oval, or even triangular. If the pencil is focussed in front of the retina, as at d, or behind it as at /, or, in other words, if the retina, in place of being at F, be in the positions G or H, there will be a luminous circle or a luminous triangular space, and many elements of the retina will be affected. The size of these diffusion circles depends on the distance from the retina of the point where the rays are focussed : the greater the distance, the more extended will be the diffusion circle. Its size will also be affected by the greater or less diameter of the pupil. Circles of diffusion may be readily studied by the following experiment, usually called the experiment of Scheiner : FIG. 5. Diagram illustrating the experiment of Scheiner. Let C be a leiis, and D E F be screens placed behind it ; hold in front of the lens a card perforated by two holes A and B, and allow rays from a luminous point a to pass through these holes; the point o on the screen E will be exact focus of the rays emanating from a ; if a were removed farther from the lens, the focus would be on F, and if it were brought near to C, the focus would then be on D. The screens F and D show two images of the point a. If, then, we close the upper opening in A B, the upper image m on F, and the lower image n on D, "disappear. Suppose now that the retina be substituted for the screens D and F, the contrary will take place, in consequence of the reversal of the retinal image, the eye be placed at o, only one image will be seen; but if it be placed either in the plane of F or D, then two images will be seen, as at TO TO, or n n ; consequently in either of these planes there Will be circles of diffusion and indistinctness, and only in the plane E will there be sharp definition of the image. To understand the formation of an image on the retina, suppose a line drawn from each of its two extremities to the nodal point and continued onwards to the retina, as in fig. 6, where the visual angle is x. It is evident that its size will depend on the size of the object and the distance of the object from the eye. Tims, also, objects of different sizes, c, d, e, in fig. 6, may be in cluded in the same visual angle, as they are at different dis tances from the eye. The size of the retinal image may obviously be calculated if we know _ FlG 6 - The Vis * al Al 8 le the size of the object, its distance from the nodal point o, and the distance of the nodal point from the pos terior focus. Let A be the size of the object, B its dis tance from the nodal point, and C the distance of o from the retina, or 15 mm. ; then the size of the retinal image A+ 15 x= p . The smallest visual angle in which two dis tinct points may be observed is 60 seconds ; below this, the two sensations fuse into one ; and the size of the retinal image corresponding to this angle is 004 mm., nearly the diameter of a single retinal rod or cone. Two objects, therefore, included in a visual angle of less than CO seconds, appear as one point. A small visual angle is in most eyes a condition of sharpness of definition. With a large angle, objects appear less sharply marked. Acuteness is deter mined by a few retinal elements, or even only one, being affected. A very minute image, if thrown on a single retinal element, is apparently sufficient to excite it. Thus it is possible to see a brilliant point in an angle even so small as of a second, and a sharp eye can see a body the - 5 th of a line in diameter, that is, about the ^^th part of an inch. (3.) The Optical Defects of the Eye. As an optical instrument, the eye is defective ; but from habit, and want of attention, its defects are not appreciated, and con sequently they have little or no influence on our sensations. These defects are chiefly of two kinds (1) those due to the curvature of the refractive surfaces, and (2) those due to the dispersion of light by the refractive media. (a) Aberration of Srjlieridty. Suppose, as in fig. 7, M A K to be a refractive sur face on which parallel rays from L to S im pinge, it will be seen that those rays passing near the cir cumference are brought to a focus at F 1 , and FlG " 7. -Spherical Aberration. those passing near the centre at F 2 , intermediate rays being focussed at N. Thus on the portion of the axis between F 1 and F 2 there will be a series of focal points, and the effect will be a blurred and bent image. In the eye this defect is to a large extent corrected by the following arrange ments : (1) the iris cuts off the outer and more strongly refracted rays ; (2) the curvature of the cornea is more ellipsoidal than spherical, and consequently those farthest from the axis are least deviated ; (3) the anterior and pos
terior curvatures of the lens are such that the one corrects,