Biophotonics_Concepts_to_Applications

In theinverted microscopeshown in Fig.8.3, the stage and the specimen are
viewed from the bottom of the instrument and the transmitted illumination of the
specimen comes from the top. When a specimen is placed on the stage, the surface
of interest that is to be examined faces downward. The advantage of this specimen
orientation is that large samples, such as a petri dish, can be positioned and viewed
easier than with an upright microscope where the size of the sample area is more
restricted. The objective lenses are mounted on a nosepiece under the stage and thus
face upward towards the specimen. To focus the microscope on the specimen, either
the nosepiece or the entire stage is moved up and down. Similar to the upright
microscope, the illuminating light source can be either a high-intensity LED or
some other source, such as a halogen, mercury, or xenon lamp.
Thestereomicroscopeis illustrated in Fig.8.4. This instrument is useful for
examining large objects, such as plants, zebrafish, drosophila (a genus of small
flies, for example, the common fruitfly), or small animals (such as mice). A key
feature of the stereomicroscope is that the viewer sees a three-dimensional erect
image. This feature is useful for performing interactive manipulations of the
specimen being examined, such as dissecting biological specimens, microinjection
of biological components into live specimens, or microscopic bonding of tissue.

8.1.2 Observation Methods

The use of microscopes can be classified according to the observation method for
various types of image enhancements. Five different microscopy observation
methods are described below and summarized in Table8.1.

Turret with

objective lenses

Movable stage

Arm

Light

source

Base

Condenser lens

Mirror

Eyepiece

Specimen holder

Fig. 8.2 Example of an
upright microscope

8.1 Concepts and Principles of Microscopy 237