cone-like bodies, arranged in a radiate manner against the inner surface 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 emanating from a luminous surface will be cut off, and that only those rays proceeding along the axis of the cone will produce 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. For anatomical details, and diseases of the eye, see Eye; the pathological aspects of vision itself are treated at the end of this article.
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 aether, 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 movements 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 produced 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 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 produced in photography. In general terms, therefore, the lower end of the spectrum may be called thermal, the middle luminous, and the upper actinic or chemical; but the three merge into and overlap one another. It may be observed that the number of vibrations in the extreme violet is not double that of the low red, so that the sensibility of the eye to vibrations of light does not range through an octave. The ultra-violet rays may act on the retina in certain conditions, as when they are reflected by a solution of sulphate of quinine, constituting the phenomenon of fluorescence. Far above the violet are the Röntgen radiations and probably others.
2. Optical Arrangements of the Eye
1. General.—When light traverses any homogeneous
transparent medium, such as the air, it passes on in a straight course
with a certain velocity; but if it meet with any other
transparent body of a different density, part of it is reflected or
returned to the first medium, whilst the remainder is propagated
through the second medium in a different direction and with a
different velocity. Thus we may account for the phenomena of
reflection of light (q.v.) and of refraction
Fig. 1.—Refraction of Light.
(q.v.) . Let ab, in fig. 1, be
a plane surface of some
transparent substance, say a sheet
of glass; a ray, cd, perpendicular
to the surface, will pass
through without refraction;
but an oblique ray, ef, will
be sent in the direction eh.
If the ray eh had passed
from a dense into a rarer
medium, then the direction
would have been eg. It
might also be shown that the
sine of the angle of incidence always bears a certain ratio to
the sine of the angle of refraction; this ratio is termed the
index of refraction. Thus, if a ray pass from air into water, the
sine of the angle of incidence will have to the sine of the angle
of the refraction the ratio of 4:3, or 43.
Before a ray of light can reach the retina, it must pass through a number of transparent and refractive surfaces. The eye is a nearly spherical organ, formed of transparent parts situated behind each other, and surrounded by various membranous structures, the anterior part of which is also transparent. The transparent parts are—(1) the cornea; (2) the aqueous humour, found in the anterior chamber of the eye; (3) the crystalline lens, formed by a transparent convex body, the anterior surface of which is less convex than the posterior; and (4) the vitreous humour, filling the posterior chamber of the eye. The ray must therefore traverse the cornea, aqueous humour, lens and vitreous humour. As the two surfaces of the cornea are parallel, the rays practically suffer no deviation in passing through that structure, but they are bent or refracted during their transmission through the other media.
From the optical point of view, the eye may be regarded as a dioptric system, consisting of various refractive media. In such a system, as shown by K. F. Gauss, there are six cardinal points, which have a certain relation to each other. These are—
(1) Two focal points: every ray passing through the first focal point becomes, after its refraction, parallel to the axis, and every ray which before refraction is parallel to the axis passes after its refraction to the second focal point; (2) two principal points: every ray which passes through the first point before refraction passes after refraction through the second, and every ray which passes through any point of a plane elevated on a perpendicular axis from the first principal point (the first principal plane) passes through the corresponding point of an analogous plane raised upon the axis at the second principal point (the second principal plane); and (3) two nodal points, which correspond to the optical centres of the two principal planes just alluded to. The distance of the first principal point from the first focal point is called the anterior focal length, and the term posterior focal length is applied to the distance of the posterior focal point from the second principal point. Listing has given the following measurements in millimetres from the centre of the cornea for the cardinal points in an ideal eye:—
| Anterior focal point | 12.8326 |
| Posterior focal point | 22.6470 |
| First principal point | 2.1746 |
| Second principal point | 2.5724 |
| First nodal point | 7.2420 |
| Second nodal point | 7.698 |
| Anterior focal length | 15.0072 |
| Posterior focal length | 20.0747 |
A view of such an ideal eye is shown in fig. 2.
The remaining measurements of such an eye are as follows:—
Radii of Curvature
|
Of anterior face of cornea |
= 8 millimetres. |
Indices of Refraction
|
Aqueous humour |
10377 = 1.3379 |
