Figure 4-2. Comparative focal lengths of elements in compound lens.

TM 9-258

aberrations or defects which are present in the single

refraction power of the former but sufficient dispersive

power to neutralize the dispersion. The result would be

(1) The refractive power of a compound lens is

that light passing through this compound lens would be

less than that of the convex lens alone. For example, if a

brought to a practical focus at a point that would be

double-convex lens of crown glass is combined with a

about double the distance of the point of principal focus

planoconcave lens of flint glass, the latter would have

of the crown lens alone (fig 4-2).

little more than half the

Figure 4-2. Comparative focal lengths of elements in compound lens.

(2) The elements are frequently cemented

at the two surfaces in contact, if the cement used, such

together with their optical axes in alinement. Two lenses

as thermosetting cement (para 2-12c), is a substance

may be cemented together as a doublet or three may be

having approximately the same index of refraction as

cemented together as a triplet, or each lens of the unit

glass.

may be mounted separately as a dialyte (fig 4-3). In the

dialyte compound lens, the inner surfaces of the

4-3.

Objectives.

a. General. The lens nearest the object in any

dissimilar faces of the two lenses cannot be cemented

together inasmuch as they are ground to different

optical system of the refracting type is called the

curvatures in order to correct for aberrations; the two

objective. Its function is to gather as much light as

lenses are separated by a thin ring spacer and are

possible from the object and form a real image of that

secured in a threaded cell or a tube with burnished

object. Objectives lenses in most optical instruments

edges. The cementing of the contact surfaces, ground

form real images (except Galilean telescope).

b. Construction. The majority of objectives are

to the same curvature, generally is considered desirable

because it helps to maintain the two elements in

constructed of two elements, a double-convex

alinement under sharp blows, it aids cleanliness, and it

converging lens of crown glass and a planoconcave flint

decreases the loss of light through reflection

4-3