Figure 5-3. Simple compound microscope

TM 9-258

tances. If it is recalled that apparent size equals actual

of the eyelens, the image is seen at infinity and no

size divided by distance to object, it should be apparent

accommodation is required. Final image may be formed

that the closer an object is to the eye the larger will be its

at any distance from the eyelens exceeding the shortest

image. The microscope uses an objective lens of

distance of distinct vision (about 10 inches). In this case,

extremely short focal length (fo) which forms a real,

the image formed by the objective lens must be within

enlarged, and inverted image of an object placed just

the principal focus of the eyepiece as in figure 5-3.

behind its principal focus. To view this image, a positive

Magnification in a microscope depends on the focal

eyepiece of short focal length (fe) is used as a magnifier.

lengths of the objective and the eyepiece and the

If the real image is at the first principal focus

distance between them.

Figure 5-3. Simple compound microscope

called either the objective or objective glass (if a mirror)

5-4. Telescopes. Most persons think of telescopes as

which usually forms a real image of the field or area

magnifiers but they may be designed to provide the eye

which the telescope can "see," and an eyelens or

with an image the same apparent size as the object or

eyepiece used to view this image.

This general

they may provide the 'eye with a smaller image which is

description is exactly the same for the microscope,

an example of reduction. The primary purpose of the

except that in the microscope the first image is actually

telescope is to improve vision of distant objects. In its

larger than the object while in the telescope the first

simplest form, it consists of two parts; a lens or mirror

image is smaller than the object.

Section II. ASTRONOMICAL TELESCOPES

cannot be brought to focus by the eye (fig 5-4). The eye

5-5. Refracting Telescopes. Refracting astronomical

must be fed either by parallel rays or rays only slightly

telescopes use lenses to form images through refraction.

diverging as if from an object no closer than the near

A positive lens alone will form only real images of distant

point of the

objects.

Such

real

images

space

5-2