Determining eye refraction
Eye disorders > The eye > Eye refraction
The term refraction of the eye is used to express the optical condition of the eye in a state of rest. There are three refracting surfaces in the eye:
And three refracting media:
These together make up the dioptric system, and may for the sake of simplicity be looked upon as equal to a convex lens of about 23mm focus. What was said about convex lenses applies equally to the eye as an optical instrument.
A ray of light falling on the cornea does not, however, follow, the simple direction we might imagine when considering the eye merely as a lens of 23 mm focus. The eye must be looked upon as a compound refracting system composed of a spherical surface and a biconvex lens. To enable us to understand the course of a ray of light through the eye, it is necessary to be acqainted with the cardinal points of this compound system. Too much space would would be occupied to explain how the position of these points is arrived
at, but it suffices to say that, having first found the cardinal points of the cornea and then those of the lens, the cardinal points of the eye will be the result of these two systems together.
The cardinal points of the eye are six in number; two principal points, two nodal points and two principal foci In the diagram above of the emmetropic eye the cardinal points of this compound system are shown, all situated on the optic axis (F A): at B we have two principal points situated so closely together in the anterior chamber that they may conveniently be looked upon as one point; at N two nodal points are located, also close together, for simplicity we shall consider them as one point; at F is the first principal focus, at A the second principal focus. We then have the following ; C, the cornea; L, the lens ; M, the macula ; O, the optic nerve; F A, the optical axis ; B, the principal point; N,
the nodal point; H, the centre of rotation of the eye, 9 mm in front of the retina; A, the second principal focus; and F, the first principal focus.
The nodal points correspond nearly to the optical centre of the refracting system and the axis ray passing through these points is not refracted ; every ray directed to the first nodal point appears after refraction to come from the second point, and continues
in the same direction to that which it first had ; the nodal points in the eye are situated about 7 mm. behind the cornea, (Fig. 24, N).
The principal points When an incident ray passes through the first principal point, the corresponding emergent ray passes through the second principal point, but the incident and emergent rays are not parallel ; the principal points are situated about 2 mm. behind the cornea. (Fig. 24, E).
The first principal focus is that point on the axis where rays parallel in the vitreous meet; this point is about 13.7 mm. in front of the cornea (Fig. 24, F).
The second principal focus is that point on the axis where parallel rays meet after passing through the eve, 22.8 mm. behind the cornea (Fig. 24, A).
A luminous paint placed above the principal axis forms its image on the retiua below this axis ; and inversely, the image of a point below the principal axis will be formed above it. If we replace these two points by an object the same things occurs, and we get an inverted image (Fig. 25) : it is essential that the method of formation of these inverted images be thoroughly understood.
From every point of an object A B C proceed divergent rays. Some of those rays coming from A pass through the pupil, and being refracted by the dioptric system, come to a focus on the retina at a; some
coming from B focus at b, and some from C at c.
In the same way come from every point of the object as divergent rays, and are brought to a focus on the retina, being exactly atthe focal distance of the refracting system, receives a well-defined inverted image.
Much has been said and written as to why images which are formed in an inverted position on the retina should be seen upright, and all sorts of ingenious explanations have from time to time been given.
The whole thing is entirely a matter of education and experience, which is supplemented and corroborated by the sense of touch. We have no direct cognizance of the image on the retina, nor of the position of its different parts, but only of the stimulation of the retina produced by the image; this stimulation is conducted by the optic nerve to the brain, producing there certain molecular changes.
We do not actually see the retinal image, but the eye receives the rays emanating from the object looked at, and we refer the sensation in the direction of these rays; thus, if an image is formed on the upper part of our retina, we refer the sensation downwards from which the rays must have come.
The great advantage of inverted images is, that for a given sized pupil a much larger retinal picture can be formed than would be the case if no inversion took place; for in the latter case all images must necessarily occupy a smaller space on the retina, than the size of the pupil.
The refraction of the eye is said to be normal when parallel rays are united exactly on the layer of rods and cones of the retina; in other words, when the retina is situated exactly at the principal focal distance of the refracting system of the eye. This condition is called emmetropia (Fig. 26, A). If parallel rays are focussed behind or in front of the retina, then the term ametropia is used, and of this there are two opposite varieties:
Hypermetropia - When the eyeball is so short that parallel rays are borught to a focus behind the retina (Fig. 26, B)
A. Emmetropic eye. B. Hyperwetropic eye. C. Myopic eye.
Myopia - When the eyeball is too long, so that parallel rays focus in front of the retina (Fig. 26, c).
Emmetropia in a strict mathematical sense is very rare.
If we represent the eye by a biconvex lens, and the retina by a screen, wbich is emmetropic when situated at the principal focus of the lens, as E, Fig. 27, we make it hypermetropic (H) by bringing forward the screen, and myopic (M) by moving it further away from the lens.
In all eyes, vision ranges from the far point or punctum remotum (which in the emmetropic eye is at infinity) to the near point or punctum proximum.
Convex lens of 23 mm. focus. Parallel rays focus at E(emohetropia) on the screen, forming a well-defined image of object from which rays come; at H (hypermetropia) they form a diffusion patch instead of an image. M (myopia), also a diffusion patch, the rays having crossed and become divergent.
The near point varies in the normal eye according, to the amount of the accommodation, receding gradually as age advances; when it has receded beyond 22 cm. (which usually occurs in the emmetropic eye
about the age of forty-five), the condition is spoken of as presbyopia.
Infinity is any distance beyond 6 metres, the rays coming from a point at or beyond that distance being parallel, or almost so.
The emmetropic eye, therefore, has its far point, or punctum reniotum, situated at infinity; the hypermetropic eye has its punctum remotum beyond infinity, and the myopic eye its punctum remotum at a finite distance.
Generally the two eyes are similar in their refraction, though sometimes there is a very great difference. One may be hypermetropic, the other myopic; or one emmetropic, the other ametropic. Anisometropia is the term used when the two eyes thus vary in their refraction.
There may be differences also between the refraction in the different meridians of the same eye astigmatism.
In all forms of ametropia the acuteness of vision is liable to be diminished. The visual acuteness usually decreases slightly as age advances, without any disease.
The acuteness refers always to central vision. The yellow spot is the most sensitive part of the retina, and the sensibility diminishes rapidly, towards the periphery. The acuteness is measured by the size of the visual angle, that is, the angle formed at the posterior nodal point, which point closely coincides with the posterior surface of the lens, and is about 15 mm. in front of the yellow spot.
In Fig. 28, let C D be an object for which the eye is accommodated. The lines C c, D d, drawn, from the extremities of the object, cross at the nodal point N.
The angle C N D will be the visual angle under which the object C D is seen. The size of the angle depends upon the distance of the object as well as upon its magnitude, and the size of the image thus formed on the retina will also depend upon the antero-posterior
length of the eyeball.
Thus an object A B, which is as large as C D, but nearer to the eye, will be seen under a larger angle, the angle A N B being greater than C N D. It is also clear that the image formed on the retina will be smaller at 1, when the antero-posterior diameter of the eye is less, as in hypermetropia, than it is at 2 in emmetropia, and that it will be larger in myopia, as at 3, where the eyeball is elongated. It is, therefore, easy to understand that a patient may be able to read the smallest type and still have some defect of refraction, unless the type be read at its proper distance
(see Fig. 35).
It is by the unconscious comparison of things of known size, and the amount of accommodation brought into play, that we are able to estimate the distance of objects, and not by the visual angle alone.
Objects must therefore be of a certain size, and it has been proved that to enable us to see a complex figure like a letter distinctly, each part of the figure must be separated from the other parts by an interval equal to not less than the arc subtending an angle of
1' at the nodal point.
It has been shcwn (Fig. 26, B) that in the hyper-metropic eye, when in a state of rest, parallel rays are brought to a focus behind the retina, so that instead of a clear, well-defined image, we get a circle of diffusion. Convex glasses render parallel rays passing
through them convergent, so that by placing a lens of the right strength in front of the hypermetropic eye, we bring forward its focus on to the retina.
In myopia (Fig. 26, C}, where the focus is in front of the retina, we succeed by concave glasses in carrying back the focus.