Monocular And Binocular Vision

Figure A2-7. Determining which eye is dominant.

a. The first shortcoming is the strain which is involved in squinting the eye and is hard for most people to endure. Another undesirable aspect is the fact that the squinting of one eye is almost always accompanied by the tension and pressure of the closed lid on the eyeball. This pressure of the eyelid upon the eyeball affects the refraction of the crystalline lens and thus has an adverse influence upon the sharpness of vision of the open eye. The third factor having an adverse effect upon the accuracy of fire is the involuntary sympathetic dilation of the pupil of the open eye in response to the closing or squinting of the other eye. Thus, when excluding the second eye from work, it is best not to squint but suppress the visual impressions of the open left eye with an opaque disc which will allow equal light intensity to affect the eye.

b. With binocular aiming, that is, with both eyes open, the line of sight still is achieved with one eye. Consequently, this method does not involve anything new in principle and therefore the shooter is not required either to learn something new or to relearn something, but must simply stop closing one eye when aiming.

5. Binocular aiming has a number of major advantages: The shooter does not have to expend the additional effort involved In squinting the eye, and this is very important when he is engaged in prolonged firings. The binocular acuity of vision is usually better than the monocular. The visual perception of one eye intensifies the total stimulus sent to the central nervous system from the visual perception of the other eye. In such aiming, the stimuli sent by two eyes are more natural than those sent by a single one.

6. All the movements of the eyeball, as well as the holding of its fixed position at moments when the glance is fixed on some object, are effected by the work of three pairs of eye muscles. During the time when the eyes are at work, including the times when the eye is aiming, these muscles are in a state of indiscernible, slight vibration or quivering. For example, when aiming a pistol and the shooter turns his head down and to the right, the eyeball turns respectively upward and inward, it is held in the least desirable position; one that requires the combined, intensified work of all three groups of muscles (Figure A2-8). When the eye muscles become fatigued, the involuntary quivering of the eyeball increases considerably and this lessens the accuracy of aiming. Therefore, the shooter must devote major attention to the position of his head when firing. He must select that firing stance in which the head position is the most natural one, with the least amount of tilt, so that the shooter does not look at the target from under his eyebrows or sideways. This results in the rapid fatigue of the eye muscles and, hence, the lessening in the accuracy of aiming.

Figure A2-8. Muscles of the Right Eye. Arrows Show the Direction in which the Eyeball turns when the Muscles designated by the numbers are contracted.

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