Figure 3-11. Farsightedness (hypermetropia)

TM 9-258

d. Astigmatism of the Eyes. This condition

observation. This is not to be confused with actual size.

A plane flying away from one will shrink in apparent size

occurs when at least one refracting surface is not

until it is only a speck in the sky. Obviously, it does not

spherical but is somewhat cylindrical (curvature not

become any smaller but, as it recedes, light rays from

symmetrical). In this case the image of a point source is

the wingtips make a progressively smaller angle until the

not a joint image but a short line image as in A, figure B-

eye no longer can separate them and the entire plane

5. Such line images will form from sources on the optical

appears as a single point. Apparent size then is

axis. Thus, visual astigmatism is different from that

inversely proportional to distance and is expressed

encountered in optical instruments (where astigmatism

mathematically as follows:

of a properly centered spherical lens system is zero on

Apparent Size = Size of Object

the axis). This condition is corrected by cylindrical or

Distance to Object

toroidal (toric) spectacles.

When two adjacent objects become so small in apparent

size that any further reduction in size results in failure of

3-11. Visual Limitations.

the eye to separate them, the angle of resolution of the

eye has been reached.

a. Apparent Size (A, fig 3-12). This is the basis

of all magnification (para 2-4 e (1)). It is measured by

the angle the object subtends at the point of

3-15