Figure 4-3. Types of objectives.

TM 9-258

(1) When the elements of the objective are of

may be magnified is, therefore, governed to a great

large diameter or when the faces of the elements are of

extent by the size of the objective. It is likewise

different curvature, the elements are not cemented but

governed by the limits set by distortion due to diffraction

are held in their correct relative positions in a cell by

(para 2-43b).

separators and a retaining ring (B, fig 4-3 and 4-35).

(2) Increasing the size of the objective beyond a

This construction permits giving the inner surfaces of the

certain point does not improve the brightness of the

two elements different values and allows greater

image appreciably because of the restriction imposed by

freedom in the correction of aberrations. An objective of

the size of the pupil of the eye.

this type is often called a dialyte or Gauss objective.

(3) Some fire-control instruments, notably tank

(2) Certain objectives are composed of three

telescopes, have objectives which are smaller than the

elements securely cemented together (C, fig 4-3) or with

eyelenses of the instruments. Most instruments of this

two of the elements cemented and one mounted

nature, however, have low magnification.

Others

separately, or with all three elements mounted

sacrifice a certain amount of the light-gathering qualities

separately. Such objectives afford a total of six surfaces

of the larger objective for compactness of instrument.

upon which the designer can secure the best possible

correction for aberrations.

4-4.

Eyepieces.

c. Relation of Objective to Optical System.

a. General. The eyepiece may be considered

(1) The size of the objective affects the amount

a magnifying glass which enlarges the image produced

of magnification that can be used in the instrument. Only

by the objective (fig 4-4). The eyepieces of modern fire-

so much light can pass through a given objective. If the

control instruments generally consist of two or three

magnification were increased, that amount of light would

lenses; one, two, or all three of which may be compound

be distributed over a larger image area and a dimmer

lenses.

image would result. The effective size to which an image

4-4