Page:EB1911 - Volume 28.djvu/148

Rh The optical constants of the human eye may be still further simplified by assuming that the two principal points and the two nodal points respectively are identical. Thus we may construct a reduced eye, in which the principal point is 2.3448 mm. behind the cornea and the single nodal point is 1.4764 mm. in front of the posterior surface of the lens. The refracting surface, or lens, has a radius of 5 mm. and is 3 mm. behind the cornea; and the index of refraction is that of the aqueous humour, or, or 1.3379.



A, summit of cornea; SC, sclerotic; S, Schlemm's canal; CH, choroid; I, iris; M, ciliary muscle; R, retina; N, optic nerve; HA, aqueous humour; L, crystalline lens, the anterior of the double lines on its face showing its form during accommodation; HV, vitreous humour; DN, internal rectus muscle; DE, external rectus; YY′, principal optical axis;, visual axis, making an angle of 5° with the optical axis; C, centre of the ocular globe. The cardinal points of Listing: H₁H₂, principal points; K₁K₂, nodal points; F₁F₂, principal focal points. The dioptric constants according to Giraud-Teulon: H, principal points united;, principal foci during the repose of accommodation; , principal foci during the maximum of accommodation; O, fused nodal points.

2. The Formation of an Image on the Retina.—This may be well illustrated with the aid of a photographic camera. If properly focused, an inverted image will be seen on the glass plate at the back of the camera. It may also be observed by bringing the eyeball of a rabbit near a candle flame. The action of a lens in forming an inverted image is illustrated by fig. 3, where the pencil of rays proceeding from a is brought 3.—Inversion by Action of a Lens. to a focus at a′, and those from b at b′; consequently the image of ab is inverted as at b′a′. The three characteristic features of the retinal image are: (1) it is reversed; (2) it is sharp and well defined if it be accurately focused on 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 focused 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 yds. the image is still brought to a focus on the retina without any effort on the part of the eye. Within this distance, supposing the condition of the eye to be the same as in looking at a star, the image would be formed somewhat behind the posterior focal point, and the effect would be an indistinct 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 circles of diffusion, the production of which will be rendered evident by fig. 4. From 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 focused in front of the retina, as at d, or behind it as at f, 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 focused: 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 studied by the following experiment, called the experiment of Scheiner:—



Let C be a lens, and DEF 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 the 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 on the point a. If, then, we close the upper opening in AB, 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. If 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 mm, or nn; 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 6.—The Visual Angle. depend on the size of the object and the distance of the object from the eye. Thus, also, objects of different sizes, c, d, e in fig. 6, may be included in the same visual angle, as they are at different distances from the eye. The size of the retinal image may be calculated if we know the size of the object, its distance from the nodal point o, and the distance of the nodal point from the posterior focus. Let A be the size of the object, B its distance from the nodal point, and C the distance of o from the retina, or 15 mm.; then the size of the retinal image x = (A + 15)/B. The smallest visual angle in which two distinct points may be observed is 60 seconds; below this, the two sensations fuse into one; and the size of the retinal image