Figure 2-62. Correction of chromatic aberration of prisms.

TM 9-258

c. Spherical and chromatic aberrations usually are

finder, is the light affected by chromatic aberration.

e. Chromatic aberration in a refracting prism is

corrected by the same two elements of a compound

corrected in much the same manner that it is in a lens.

lens.

In some lenses, the negative element is

Two prisms made of different kinds of glass are

planoconcave (only one surface ground spherically). In

cemented together (fig 2-62). The prism having the

this case, the adjacent surfaces are ground to

larger refracting angle and which is to be the means of

compensate for these two defects. In others, the inner

refracting light from its normal path is made of crown

curves of the lenses are calculated to eliminate

glass. The prism having the smaller refracting angle is

chromatic aberration and the outer curves to remove

made of heavy flint glass which has a large dispersion of

spherical and other types of aberration.

d. The majority of prisms employed in fire-control

colors. The flint prism, by reason of its large dispersion,

neutralizes the dispersion caused by the crown prism

equipment are used to reflect light; some are used to

without entirely neutralizing the deviation of the path of

refract light. When light is reflected by a prism, there is

light. This compound prism is known as an achromatic

no chromatic aberration because the light rays are not

prism.

divided up or dispersed. Only when light is refracted by a

prism, as through a measuring wedge of a range

Figure 2-62. Correction of chromatic aberration of prisms.

b. A perfect lens refracts rays from a point of light

2-40. Astigmatism in Lenses.

to a sharply defined point of light on the image. The

a. Astigmatism is a lens aberration which makes it

useful rays which form the image are refracted as a cone

impossible to get images of lines equally sharp when

(fig 2-63). Each cross section of one of these cones is

these lines run at angles to one another. This optical

circular in form; each successive circle becoming smaller

defect is found in practically all but relatively complex

until the focal point is reached

lenses known as anastigmats which are designed to

practically eliminate this condition.

2-46