Figure 3-4. Suspension and action of crystalline lens .

TM 9-258

d. The crystalline lens is double-convex and the

necessary in most cases. Bifocals contain lenses having

a small section ground to a higher power through which

front surface is flatter than the rear (fig 3-2). When the

the user looks at near objects.

eye is relaxed (B, fig 3-4) the lens of a normal eye will

e. The image formed on the retina is inverted, but

focus upon distant objects. To increase the refractive

we do not see the image inverted, as there is simply a

power of the lens, for focusing the eye upon a close

correspondence between the retina and external

object, the surfaces are made more strongly curved,

directions.

more convex by the compressive action of the tense

muscles of the ciliary body (C, fig 3-4). This process is

known as accommodation, permitting the normal adult

3-6. Response Mechanism.

eye to view near objects as close as 25 centimeters

a. The response mechanism of the eye, the area

(about 10 inches). An object viewed at this distance is

on which images are formed and examined consists of

said to be at the near point of the eye. The ability to

the retina (fig 3-5) or thin inner coat of the eyeball which

accommodate usually decreases as a person gets older

is sensitive to light. The cause of light sensitiveness of

making the wearing of bifocals

this area is an almost infinite number of visual cells.

They are connected by nerve fibers to the optic nerve

3-5