The United States Army Marksmanship Unit Training Manual

Figures A2-4. Schemes Showing the Refraction Rays in the Eyes: (a) Normal, (b) Nearsighted, (c) Farsighted.

c. The eye is farsighted if the rays entering it are focused in back of the retina (Figure A2-4(c)). This can occur either as a result of the eye having a weak refracting force, or that the eye is too short from front to back or by a combination of both factors. In such an instance, in order for the rays to focus upon the retina, they must enter the eye in a converging beam, and therefore a farsighted eye sees near objects worst of all. This type of eye is harder to correct but eyeglasses help it overcome the difficulty. Shooter’s suffering from farsightedness will see the sights very poorly. The characteristic complaint of farsighted persons is that the rear notch seems to fuse with the front sight.

d. An indistinct, hazy image of objects on the retina can also result from astigmatism of the eye.

(1) An eye in which the refracting surfaces of the cornea and crystalline lens do not have a perfectly spherical form is called astigmatic. When an eye is astigmatic, parallel rays entering the eye cannot produce an exact focused image on the retina. The refraction of the light rays at various points in the eyeball occur at different angles. As a result, the eye possesses not a single principal focus, but several foci, which are located at various distances from the retina; that is why the image on it is indistinct and uneven (Figure A2-5). It must be said that certain strange phenomena frequently observed under practical conditions of marksmanship (for example, when two shooters are firing the same pistol, with an identical sight setting, and the centers of impact differ sharply from one another) are, in all probability, connected with astigmatism of the eye.

Figure A2-5. Scheme showing the Refraction of Rays in an Astigmatic Eye.

(2) One can easily be convinced of the existence or absence of astigmatism by making use of a circle chart (Figure A2-6). For this purpose it is necessary to look with one eye from the distance of best vision (approximately 10 feet) at a disk on which concentric circles are drawn. If the person has astigmatism, only certain areas of the disk will be clearly visible and the remaining areas will seem hazy.

Figure A2-6. Chart for Discovering Astigmatism.

4. Correction of Defects: If even insignificant defects in vision are discovered, it is necessary to wear corrective eyeglasses when firing, since the excessive accommodation of the eye resulting from aiming will greatly fatigue vision and this can lead to a still greater decrease in its accuracy. It must also be kept in mind that when eyeglasses for firing are chosen by the ordinary method, that is, in an optometrist’s office, the choice is not completely satisfactory for the shooter. It is very desirable when selecting the lenses to check them immediately on the pistol range, to make sure that one can see well the sight alignment located at arm’s length distance away from the eye. This selection is linked not so much with the determination of the lens dioptrics, as with the determination of the quality of their grinding, since all defects will make themselves known quickly during such a check.

When wearing shooting glasses (including those with filter lenses) it is necessary to make sure that the line of sight runs perpendicular to the surface of the lens and through its center, since the central portion of the lens is usually ground considerably better and therefore has less distortion of the vision. To hold the lenses perpendicular to the line of sight requires no change in the ordinary level placement of the head when assuming the firing position.

It is necessary to dwell on one more peculiarity of our eyes which has tremendous importance in aiming — the existence of monocular and binocular vision.

1. Vision with one eye is called monocular and vision with two is called binocular. The fact that a person has two eyes does not always mean that he also has binocular vision. There are instances when the eye which has the poorest vision is not included in the act of vision and the person actually uses just one eye, the better one. The dominance of one eye over the other also occurs, even when both eyes possess identical sharpness of vision. The eye that the person prefers to use is called the dominant, or directing eye. There exists a very simple method by which one can determine which of the eyes is the stronger.

2. In order to determine which is the dominant eye the shooter must hold his hand out slightly making a ring out of the fingers and thumb, and look through it in such a way that some small object can be seen by both eyes (Figure A2-7). Then, by alternately closing one eye then the other, it is necessary to see whether the object stays within the ring or leaves it. The dominant eye is the one with which the shooter sees the object as un-shifted, remaining in the ring. In most people the dominant eye is the right one.

3. The protracted work of one eye (for example, by laboratory assistants, microscopists) contributes to the fact that the eye used becomes the dominant one. This naturally pertains also to shooters, who, when aiming, use one eye. The right eye is, in the overwhelming majority of cases, their dominant one.

Figure A2-7. Determining which eye is dominant.

4. In the past the shooter has usually been instructed to squint his left eye and aim the pistol at the target with his right eye. During subsequent instruction it is no longer necessary to continue this device of closing the eye, since it has its major shortcomings, which are attested to by numerous instance of medical research.

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.