Figure 2-58. Spherical aberration reduced .

TM 9-258

Figure 2-59. Effect of compound lens on spherical

aberration .

g. Spherical aberration is minimized by "bending"

the lens. Bending is accomplished by increasing the

curvature of one surface and decreasing that of the

other. This tends to eliminate spherical aberration while

retaining the same focal length. In telescope design, it is

common practice to minimize spherical aberration in yet

another way by placing the greater curvature of each

lens toward the parallel rays so the deviation at each

surface is nearly equal as in B, figure 2-58. The angles

of incidence and emergence must be equal for minimum

spherical aberration. In accord with this rule, telescope

objectives always are assembled with greater curvature

(the crown side) facing forward.

2-39.

Chromatic (Color) Aberration.

a. White light consists of all colors. Upon being

refracted through a prism, white light is dispersed into

rays of different wavelengths forming a spectrum of

Figure 2-58. Spherical aberration reduced .

various colors (para 2-27). The rays of different colors

are refracted to different extents; red undergoing the

f. In fire-control instruments, spherical aberration is

least refraction and violet the most. Inasmuch as a lens

commonly eliminated by the use of a convergent and a

may be thought of as being composed of an infinite

divergent lens cemented together to form a single

number of prisms, this dispersion also exists where light

element known as a compound lens (para 4-2). The

is refracted through a lens. This produces an optical

compound lens approximately corrects spherical

defect, present in every uncorrected single lens, known

aberration because the concave curves of the divergent

as chromatic aberration or chromatism. The violet rays

lens neutralize the positive aberration of the convex

focus nearer the lens than the red rays (fig 2-60) and the

curves. The refractive power of the combination is

rays of the other colors focus at intermediate points.

retained by the proper choice of indices of refraction for

Thus, such a lens would have a different focal length for

each of the two lenses. A lens in which spherical and

each color and the image would be fringed with

other aberrations have been minimized or eliminated is

said to be aplanatic (fig 2-59).

2-44