Page 21 - Fundamentals of Light Microscopy and Electronic Imaging

4 FUNDAMENTALS OF LIGHT MICROSCOPY

mass closer to the lab bench and are therefore less sensitive to vibration. However,
there is some risk of physical damage, as objectives may rub against the bottom sur-
face of the stage during rotation of the objective lens turret. Oil immersion objectives
are also at risk, because gravity can cause oil to drain down and enter a lens, ruining
its optical performance and resulting in costly lens repair. This can be prevented by
wrapping a pipe cleaner (the type without the jagged spikes found in a craft store) or
by placing a custom fabricated felt washer around the upper part of the lens to catch
excess drips of oil. Therefore, despite many advantages, inverted research micro-
scopes require more attention than do standard upright designs.

APERTURE AND IMAGE PLANES IN A FOCUSED,
ADJUSTED MICROSCOPE

Principles of geometrical optics show that a microscope has two sets of conjugate focal
planes—a set of four object or field planes and a set of four aperture or diffraction
planes—that have fixed, defined locations with respect to the object, optical elements,
light source, and the eye or camera. The planes are called conjugate, because all of the
planes of a given set are seen simultaneously when looking in the microscope. The field
planes are observed in normal viewing mode using the eyepieces. This mode is also called
the orthosocopic mode, and the object image is called the orthoscopic image. Viewing the
aperture or diffraction planes requires using an eyepiece telescope or Bertrand lens, which
is focused on the back aperture of the objective lens (see Note and Fig. 1-3). This mode of
viewing is called the aperture, diffraction, or conoscopic mode, and the image of the dif-

Figure 1-3
The back aperture of an objective lens and a focusable eyepiece telescope.