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 should be done. There are three things that every healthy eye does: Blink, center its attention (called Central Fixation) and shift.
1. Blinking, the first habit of normal sight an involuntary action. The blink is the quick, light, easy closing and opening of the eye, and it is done intermittently by every normal eye. The rate of blinking varies with people and also varies with the use an eye is put to. You blink more, for instance, when you look at something brilliant than you do when you look at something soft in tone.
a. Frequently the dividing point between a normal and abnormal pair of eyes is its impulse to blink under a given situation. If the eyes are perfectly normal, they will blink; suppression of the act of blinking shows a tendency to become abnormal.
b. The action of the eyelids in blinking is most essential to normal eyes and sight. The fluid that keeps the eyes moist is produced by a small gland called the lacrimal gland under the outer portion of the upper lid. When one blinks, this fluid is washed down and over the eyeball and keeps the eye moist.
This moisture has several functions:
(1) There is a definite antiseptic and cleansing action of the fluid.
(2) The brilliance of the eyes and their ability to reflect light are largely due to the fluid on their surface.
(3) The fluid is essential to the cornea, which is the small translucent front part of the eye. Since the cornea has no blood vessels, it needs this fluid to keep it moist or it may develop corneal ulcers.
(4) When particles of foreign matter get into the eye, the lacrimal fluid tends to float them off, while on an eye that is dry, the particles may stick and imbed themselves.
(5) In cold weather, frequent blinking tends to keep the eye warm. An eyeball can be very uncomfortable when cold.
(6) In strong wind or when the weather is very dry, blinking comforts and protects the eye. Under these conditions, one should blink frequently, almost continuously, because the fluid is lost so rapidly.
(7) In the short interval of blinking, the muscles of the pupil have a chance momentarily to relax their tension.
(8) Blinking also enables the eye to move slightly and thus allows the recti muscles to make the small amount of movement essential to their well-being, since motion is necessary to the health of any muscle.
(9) The circulation of the lymphatic fluid around the eye is aided by blinking, and the eye is strengthened by this good circulation, just as any body is benefitted by keeping the circulation of the blood active around it.
c. Blinking is not an interruption of continuous vision. Continuous vision is the illusion that a normal eye produces, authentic in effect but nevertheless an illusion. When an image falls on the retina, there is another image or an after-image produced. In other words, the image remains on the retina for a short period longer than the image is kept before the eye. It is as if your image in the mirror, stayed there a moment after you had gone away.
Thus, it is not necessary for the eye to be seeing actively all the time in order to produce the illusion of seeing constantly. In fact, nothing in the body works more than half time or so much as half time. More than half of the time of every organ is consumed in the repair and replacement of its own tissue and the elimination of its waste products.
d. The frequency of the visual impressions made by the eye is between thirty and forty images per second in the average person. Therefore the blink does not interfere with constant vision. It is possible for the eye to blink so frequently that the eye is closed half of the time and yet it will see as much as if it were open all the time.