Section III. BINOCULAR (TWO-EYED) VISION STEROVISION

TM 9-258

b. The light sensitive elements or visual cells are of

e. One small portion of the retina is insensitive to

two distinct types called rods and cones (fig 3-5). The

all light. This is an area known as the blind spot or optic

disk. It is where the optic nerve enters the eye and is

rods are cylindrical-shaped and longer than the cones;

the shorter cones are bulb-shaped. All are so small that

located slightly to one side of the macula. In most

they must be magnified to 300 times their actual size to

instances, this blind spot has no effect on vision because

be visible. While the mechanism of the eye has not been

the mind has learned to ignore it.

f. The optic nerve carries visual sensations to the

fully explained it appears that the cones provide daylight

vision, where light intensities are high; the rods are more

brain. This nerve is really a cable of nerves through

sensitive to light, and give twilight vision, where light

which the rods and cones activated by light images send

intensities are low. The cones give clear sharp vision for

messages to the brain which result in what is termed

seeing small details and the distinguishing of colors; the

sight.

rods detect motion.

g. Foveal vision is much more acute than

c. A slight depression in the retina on the visual

extrafoveal vision and the muscles controlling the eye

axis of the eye is called the macula or yellow spot. It is

always involuntarily rotate the eyeball until the image of

about 2 millimeters in diameter and contains principally

the object toward which the attention is directed falls on

cones and only a few rods. At the center of the macula

the fovea.

h. The outer portions of the retina serve to give a

is the fovea or fovea centralis, where the retina is much

more highly developed than elsewhere, s small area

general view of the scene and to warn of an object

about 0.25 millimeter in diameter containing cones alone.

approaching within the field, while the fovea enables the

This area of the retina, therefore, gives the sharpest

object of chief interest to be examined minutely. If the

detail and the best appreciation of color. No fibers of the

eye is directed at a particular point on a printed page only

optic nerve overlie it.

the words close to that point are seen distinctly.

d. The rods are stimulated by a substance known

i. An impression registered upon the retina and the

as visual purple to increase their sensitiveness of

optic nerve persists, at least as a mental effect, for an

subdued illumination. This substance builds up and

appreciable time (about 1/15 sec) after the stimulus has

accumulates during the time an eye is becoming

been removed. Thus, the glowing end of a whirling stick

accustomed to darkness. An eye affected in this manner

gives the impression of a streak of light. This is known

as persistence of vision. It is this effect which is the chief

is termed a dark-adapted eye. The visual purple is

bleached out by light. A different substance serves the

aid in securing the illusion of smooth motion in moving

cones in the same way to produce what is known as a

pictures so that we are not aware of the gaps between

light-adapted eye.

successive pictures.

Section III. BINOCULAR (TWO-EYED) VISION STEROVISION

between the pupils of the eyes which enables the eyes to

3-7. General.

see objects from slightly different angles. Eyes normally

a. Binocular vision is coordinated two-eyed vision.

are about 62 millimeters apart. This distance between

The two eyes of a person are normally identical and they

the eyes is called the interpupillary distance and is

work together as a team.

The muscles used in

abbreviated "ipd".

adaptation and accommodation of the eyes are

c. The ability to record with each eye a slightly

sympathetic in action; that is, they tend to dilate,

different picture or image of the same object often

contract, and focus together. Both eyes usually meet

enables the observer to see more of one side of a given

with the same light conditions and are turned to converge

object with one eye than with the other. Together, the

on the same object or field of view. The two images

two eyes see farther around the object than one eye can

received by the eyes are not seen separately but are

see. They get a better impression of the shape of the

fused by the brain into a single image.

object, its depth, and its position with relation to other

b. Stereoscopic vision, stereovision, or stereopsis,

objects. This ability is increased through the use of

as it is called, is inseparably associated with binocular

certain firecontrol instruments which increase the virtual

vision because it is the result of seeing with both eyes.

distance between the pupils of the eye of the observer by

Stereovision is the power of depth perception; the ability

optical means.

to see in three dimensions. This power is due to the

spacing

3-7