ANNEX II. OPTICAL PROPERTIES OF THE EYE RELEVANT TO SIGHTING
A. GENERAL
The principal difficulties which confront the shooter during aiming are determined by the inherent characteristics of the organ of vision — the eye — and its work as an optical apparatus during the aiming process.
It is well known that the aiming process makes very exacting demands upon the vision, since consistency and degree of accuracy are directly dependent upon the sharpness of vision and the conditions determining them. Therefore, it is necessary for the shooter to have a knowledge of certain of the optical properties of the eye. He must know the degree and the conditions under which the optical imperfections of the eye can affect the accuracy of aiming.
Figure A2-1. Horizontal Cross Section of the Human Eye (Right Eye)
1. Cornea (transparent, grasslike portion of coat of eyeball).
2. Conjunctiva (mucous membrane which lines eyelids and is reflected onto eyeball).
3. Ciliary muscle (smooth muscles, controlling alteration of crystalline lens).
4. Muscle of eyeball.
5. Space posterior to crystalline lens.
6. Sclera (toughest of the three membranes, forming the outer protective and supporting layer of the eye ball).
7. Choroid (vascular coat of the eye).
8. Retina (innermost tunic of the eye, containing receptors, rods and cones).
9. Optic disk (blind spot of retina).
10. Optic nerve (transfers images from retina to visual nerve centers located in the brain).
11. Anterior chamber (filled with aqueous humor).
12. Iris (opening in center is called the pupil, contains groups of smooth muscles that dilate and contract pupil).
13. Posterior chamber (filled with aqueous humor).
14. Suspensory ligaments (zonule of Zinn).
15. Crystalline lens (transparent biconvex body enclosed in transparent sheath, suspended from ciliary body by suspensory ligaments).
16. Vitreous (transparent jelly-like substance contained within transparent membrane close to retina).
17. Fovea central is (a pit in the middle of the macula lutea).
18. Macula Lutea (point of clearest vision, contains greatest number of cones).
B. OPTICAL PROPERTIES OF THE HUMAN EYE
The human eye as a visual analyzer makes it possible to distinguish rather accurately between colors, form, dimensions, degree of illumination, and the location of objects in the immediate environment (Figure A2-1).
1. The forward portion of the eye, which is turned toward the light, contains a light-refracting apparatus which transmits the image to a light-sensitive membrane — the retina; this apparatus consists of a system of refracting media and surfaces — the cornea, the crystalline lens, the aqueous humor, and the vitreous humor filling the optical cavity. (The light-refracting apparatus also includes the ciliar humor and the iris, which has an opening, the pupil, in the center of it.)
2. The degree of illumination, the form and location of the objects surrounding us are perceived by the internal light-sensitive membrane — the retina — which is linked by means of the optic nerve to the cerebral cortex. In order to obtain the correct visual perception of any object, the image of it upon the retina must be sharp. This is achieved as a result of the eye’s ability to adapt its light-refracting system and thus to obtain on the retina a sharp image of objects located at varying distances from it.
3. In our eye, the role of photographic lens is played by the crystalline lens which is a transparent, biconvex body similar in form to an ordinary lens. When the eye observes objects located at varying distances, the curvature of the crystalline lens changes, as a result of which the eye’s optical system adapts by reflex and very rapidly to the perceiving of objects located at varying distances from us. As a result, the image produced on the retina is a sharp one and this makes it possible to perceive correctly and sharply the form and outlines of the objects surrounding us. This ability of the eye to adapt to the viewing of various objects located at varying distances from the eye by means of changing the curvature of the crystalline lens (changing the refraction) is called accommodation.
4. Consequently, the human eye is constructed in such a way that it is not able to see sharply, simultaneously, objects located at varying distances from it. Therefore, it is not possible when aiming, to see with identical sharpness the sight alignment and the target which is located at varying distances from the shooter’s eye. Understanding this, one must not strain the vision excessively in vain attempts to see everything sharply at the same time.
5. The normal eye in the state of rest is set up to perceive distant objects, that is, it is set to infinity. In order to switch the vision to perceive objects located close by, it is necessary to exert a definite muscular effort. The mechanism of accommodation lies in the fact that the ciliary muscle contracts, and the crystalline lens takes on a convex form, thus increasing its refraction.
As a result, when aiming, one must not abuse the eye by shifting the glance with excessive frequency, from one point of clear vision to another. For example, a shift of focus from the rear notch and the front sight to the target, and back again. The protracted muscular effort expended under such conditions leads to the rapid and considerable fatiguing of the oculomotor muscles. For the same reason, the shooter must not aim for long sustained periods. He must make sure that in the intervals between sequences of aiming, he does not concentrate his glance on some object, but looks momentarily into the distance “with an absent-minded stare” in order to rest his eye muscles.
6. When there is a change in the force of natural illumination, the level of the eye’s light-sensitivity changes and the eye adapts to the different amount of light entering it. A role similar to that played by the diaphragm in a camera is played by the pupil, the opening or aperture in the middle of the iris. Under the action of various eye muscles, the diameter of the pupil can be made narrower or wider. It is this action which regulates the amount of light entering the eye and which improves the depth of focusing of the image upon the retina when the pupil becomes narrower. The question of the speed at which the pupil reacts to a change in illumination is also deserving of attention. When the light changes to greater brilliance, the pupil contracts much more rapidly than it expands after finding itself again in conditions of lesser brilliance. For example, the contraction of the pupil to the stable level of average light intensity takes about 5 seconds, but the process of its reverse dilation after the stimulus created by low intensity light requires about 3 minutes. From this the shooter must also make the corresponding conclusions: in order to preserve the eye’s working efficiency without reducing the accuracy of aiming, before or during aiming, one must not look at brightly illuminated objects or, moreover, subject the eye to the action of sharp transitions from light to shadow. In the intervals between shots, one must not rest the eyes by closing them. Between shots, it is necessary to rest the eyes, but the best way is to look at distant dull surfaces having even tones of gray, green, or blue.
C. FUNCTION OF THE HABITS OF THE NORMAL EYE
All shooters should make a conscious effort to improve the condition of their eyes in the intervals when they are not actually aiming by allowing the habits of normal sight of function. The following will give an idea of how this
