If you do not shift it, you will stare, and staring is one of the worst and commonest forms of eye strain.
a. Shifting is a normal function and is normally done unconsciously. The frequency with which your eyes shift varies with the type of demand upon the eyes; for instance, looking at a book or watching a tennis match. The book is stationary and the eyes do not tend to move, while the tennis balls and players are constantly in motion so the eyes must move continually in order to follow them.
b. But, in any event, shifting should be as frequent as possible. The time required for an image to register on the retina, about 1/50 of a second, allows for a great frequency of shifting with no loss or interruption of vision.
c. People who are inclined to look at one area too long, and every abnormal eye does this, would benefit both in vision and in eye comfort if frequent shifting from the point being looked at is consciously practiced. If your vision is abnormal; without wearing your glasses look at a word, then look at a word three word spaces beyond it; then back up to the original word. Do this until both words become clear. Be relaxed while you practice.
d. Or, if your vision is good; look at the moon and while blinking frequently, shift your vision from one point to another on the moon. Do this a number of times and the moon will stand out much more clearly and appear in its true form as a solid spherical body with depth and shape instead of a flat disc.
e. Shifting is both voluntary and involuntary in character. The involuntary shift is continuous, automatic and very slight. This movement is not visible and is believed to correspond in frequency with the rate of image production in the retina.
f. There is in every muscle a faint tremor, since muscle tone is not a constant factor but is a rapid succession of contractions producing a relatively steady muscle pull. And, since the eyes are held in position by muscles and all focusing is produced by these muscles, the eyes are naturally subject to all conditions that muscles produce incidental to their normal functioning.
g. When the eye is relaxed, the voluntary shifting is frequent and the movement is short in scope. The tense eye can make a large movement, but it requires relaxation and normality for an eye to keep shifting in relaxed condition with a very small movement. This is true of all muscles — the finer the movement, the better trained and the more relaxed must be the muscle. When an eye is strained and the vision is abnormal, practice in shifting frequently will give relief from the strain and produce improvement in the vision.
h. An exercise that accomplishes this is to focus definitely on each word and consciously shift to the next one. A few minute’s practice each day will make this an unconscious habit.
i. Normal shifting is absolutely essential to normal sight. Loss of vision is frequently in direct proportion to the loss of motion.
4. In addition to acquiring the three habits described above, a shooter may find it desirable to strengthen his tolerance for light. This may be done as described in the following paragraphs:
a. Sunlight is very beneficial to the eyes. It both relaxes and stimulates. But it is necessary to know how to use the sunshine to get the most out of it. Sunlight directly on the eyes may cause great damage. The eye can be strengthened in its light tolerance by judicious exposure to light. One of the most effective and simple ways of strengthening the eyes is to expose them to the sun’s rays in the following manner:
(1) Close the eyes lightly as the face is turned directly toward the sun.
Keeping the eyes closed, slowly turn the head from side to side. Keep this up for four or five minutes. Then, when the eyes are relaxed from the heat of the sun and the motion of the head, they may be opened, but only momentarily, and when the head is turned to the side. The eyes must not look directly at the sun but may look near it. Make no effort to see, and open the eyes only in flashes. As this exercise is continued, and the eyes become accustomed to the increased light, the glance may be directed closer and closer to the sun.
(2) By doing this with regularity on successive days and for a gradually increasing length of time, any eye will be strengthened and its vision improved.
b. The eye is admirably equipped to protect itself and function under widely varying light conditions. When the natural protective mechanism is used, as just outlined, strong light will be handled easily by the eye.
D. OPTICAL IMPERFECTIONS OF THE HUMAN EYE
As a result of various optical imperfections of the eye, the images of objects on the retina have edges which are not completely sharp, or are to a degree totally fuzzy. As a consequence, there exists a certain limit of varying sensitivity of our eye which determines the sharpness of vision. It must be said that sharpness of vision, in and of itself, is inconstant. It has a certain variable value which depends upon the degree and the conditions under which, the optical imperfections of the eye have a noticeable effect. Therefore, the shooter must know, at least in overall features, the conditions which influence the sharpness of vision and thus the degree of accuracy of aiming.
As an optical instrument, the eye has inherent in it, the phenomena of aberration and diffraction of light.
1. Spherical aberration is a function of the eye in which rays of light falling upon the crystalline lens are refracted differently and are not focused at a single point. The extreme outer rays are refracted more strongly than the central ones (Figure A2-2). As a result of spherical aberration, a beam of parallel rays entering the eye is focused on the retina not in the form of a sharp image, but in the form of a circle of light diffusion. The size of the circle of light diffusion resulting from spherical aberration is in direct proportion to the size of the pupillary opening. It is obvious that the sharpness of the image is increased if one eliminates extreme rays. Consequently, as the pupillary opening contracts, the sharpness of the image of the object upon the retina increases.
The degree to which spherical aberration can hinder the seeing of objects sharply, and to which the sharpness of the image depends upon the size of the pupillary opening, can be convincingly shown to the shooter by means of a simple example. Small orienting marks and objects which can be distinguished only with difficulty during overcast weather become incomparably more discernible if one looks at them through a small peep hole which, in this instance, fulfills the role of an artificial pupil.
Figure A2-2. Phenomenon of Spherical Aberration.
2. The phenomenon of light diffraction lies in the fact that light rays passing through small openings, particularly through the crystalline lens; seem to bend (figure A2-3) and produce on the retina an image not in the form of a single sharp point, but in the form of a circle surrounded by a number of concentric light rings of decreasing sharpness. This occurs as a result of the wave nature of light.
a. As the pupillary opening decreases, the diameter of the diffraction ring of light diffusion increases. The diffraction rings around the images have a noticeably telling effect only when there are extremely small dimensions of the pupil, and this, as we can see, is a certain opposite of the phenomenon of spherical aberration. The phenomenon of diffraction makes itself felt when there is solar illumination from the front and the sun in shining into the eyes; when there are bright patches of sunlight on the horizontal surfaces of the front and rear sights, etc.
Figure A2-3. Phenomenon of Diffraction of Light on the Pupil.
b. The operation of the eye as an optical apparatus is also harmed to a certain degree by the light diffusion occurring with in it. It is especially discernible when one views brightly illuminated objects located against a dark background. The effect of light diffusion in the form of a more or less noticeable radiation, covering the field of vision, is caused by media which do not possess absolute transparency — the crystalline lens and the vitreous humor. The light diffusion in the optical media is responsible for the halos of light. They are especially noticeable where the targets are strongly illuminated by sunlight. In such an instance, the white background of the target casts a sharp reflection and causes a considerable light diffusion in the optical media. This causes a blinding effect. Both the bull’s-eye, perceived by the eye in the form of a gray spot with indistinct edges, and the front and rear sights are perceived with unclear outlines.
c. It is obvious from what has been said that the amount of light diffusion from spherical aberration is in direct proportion to the size of the opening of the pupil, and the amount of light diffusion from diffraction is in inverse proportion to the size of the opening of the pupil, and thus it is not possible to eliminate these types of diffusion completely. As a result of this inverse dependence of the effects of aberration and diffraction upon the size of the pupil, the best conditions of sharp vision correspond to a certain average size of the pupillary opening — a
