Polaroid MicroCam Manuel d'utilisateur télécharger pdf (Page 13)

4 Align the substage condenser by

centering the image of the field

diaphragm, using the condenser’s radial

centering screws (21). If necessary,

refocus the condenser once again, to

keep the field diaphragm in sharp focus

with the specimen image.

5 Open the field diaphragm until it is just

outside the field of view or, for

photography, until it is just beyond the

area of the film format (22). Do not open

it any farther, since this could cause flare

and a loss in contrast.

Adjusting the aperture diaphragm

The simplest method of adjustment is to

close the aperture only far enough to

notice a loss of brightness. A more

precise method of adjustment is to

remove the eyepiece (or camera) and

look down the tube. The correct setting

will generally be such that the circle of

light within the diaphragm blades has a

diameter about 2/3 to 3/4 that of the entire

light disc (23).

The proper setting is critical for good

photomicrographs; the diaphragm should

be closed down sufficiently to provide the

desired image contrast, but not so far as

to cause unwanted diffraction patterns

that result in a loss of detail.

The light that forms the microscope

image may be either direct (absorbed) or

deflected (refracted/diffracted). The aper-

ture diaphragm controls the “mix” of

direct and deflected light. The correct set-

ting is somewhat dependent on the type

of specimen, and how that specimen uses

direct and deflected light. For example:

• A diatom uses mainly deflected light, and

not much direct light. The aperture

diaphragm should be closed down to

about 25%, to produce the best image.

• Stained tissue uses both direct and

deflected light; an aperture diaphragm of

50% to 90% will produce a good image.

• A stained blood smear uses mainly direct

light, very little deflected light. An

aperture diaphragm setting anywhere

between 25% and 90% will produce a

good image.

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