they have nearly the same refractive index. The further these refractive indexes
are from each other, the greater the contrast between the specimen and the field
of view.
Unfortunately, refractive indexes of the specimen and the field of view are
fixed. However, you can tweak the refractive index of the specimen by using a
stain. The stain adheres to all or part of the specimen, absorbing additional light
waves and increasing the difference between the refractive indexes of the speci-
men and the field of view. This results in an increase in the contrast between the
specimen and the field of view.
Oil Immersion
A challenge facing microbiologists is how to maintain good resolution at mag-
nifications of 100×and greater. In order to maintain good resolution, the lens
must be small and sufficient light must be reflected from both the specimen
and the stain used on the specimen. The problem is that too much light is lost;
air between the slide and the objective prevents some light waves from pass-
ing to the objective, causing the fuzzy appearance of the specimen in the ocu-
lar eyepiece.
The solution is to immerse the specimen in oil. The oil takes the place of air
and, since oil has the same refractive index as glass, the oil becomes part of the
optics of the microscope. Light that is usually lost because of the air is no longer
lost. The result is good resolution under high magnification.
TYPES OF LIGHT COMPOUND MICROSCOPES
There are five popular light compound microscopes used today (see Table 3-3).
Bright-Field Microscope
The bright-field microscope is the most commonly used microscope and consists
of two lenses. These are the ocular eyepiece and the objective. Light coming from
the illuminator passes through the specimen. The specimen absorbs some light
waves and passes along other light waves into the lens of the microscope, causing
a contrast between the specimen and other objects in the field of view. Specimens
that have pigments contrast with objects in the field of view and can be seen by
using the bright-field microscope. Specimens with few or no pigments have a low
contrast and cannot be seen with the bright-field microscope. Some bacteria have
low contrast.
CHAPTER 3 Observing Microorganisms^57