Figure B-22. Nearsightedness (myopia) .

TM 9-258

Mount - A device for attaching or supporting an

Myopia - An eye defect which does not permit

optical instrument. It may be may not be provided with

distant objects to be seen clearly, nearsightedness.

means for elevating and traversing the instrument,

It is the result of eyes that do not conform to standard

calibrations to indicate degree of movement, and means

measurements, having too much optical depth or too

and gages for leveling. This term is applied also to

great curvature of surfaces. The image tends to be

means of securing an optical element in an instrument.

focused before it reaches the retina and is indistinct. It is

Muscular imbalance - See heterophoria.

corrected by a diverging lens (fig B-22).

Figure B-22. Nearsightedness (myopia) .

Navy mil - See mil.

Near point - Closest point of clear vision with the

unaided eye.

Nearsightedness - See myopia.

Negative lens - See diverging lens.

Newton's rings - A series of concentric colored or

bright and dark circles seen when positive and negative

(convergent and divergent) surfaces of nearly the same

curvature are pressed together.

It is caused by

interference of light rays. It is used to test lenses. The

more nearly the surfaces are matched, the greater the

distance between the rings and the larger and more

regular the circles (fig 2-70).

Figure B-23. Normal and oblique .

Night blindness - A condition of the eyes (and

possibly the health) of a person which does not permit

Normal erect - See erect image.

him to see as well at night or with poor illumination as the

Normal inverted - See inverted image.

average person.

North - See Magnetic North, True North, Y-North

Night glasses - An optical instrument for use at night

(Grid North).

having an exit pupil at least 7 millimeters.

Object - The principal thing imaged by the optical

Normal - An imaginary line forming right angles with

system. It may be part of large object, an entire object

a surface or other lines, the perpendicular. It is used as

plus its immediate surroundings, or a number of objects.

a basis and reference line for determining angles of

It is a general term used for the sake of convenience.

incidence, reflection, and refraction. In figure B-23, the

normal is perpendicular or at right angles to all lines lying

in the plane and passing through point 0.

B-22