28 PART I Basics of Microscopy
Compound Microscopes – Clever
Combination of Lenses and Light
The simplest version of a microscope has only a single optical system and basically
works like a magnifying glass, which collects and focuses the light. To get higher
magnifications than 25-fold you can either increase the curvature of your lens
(which is limited) or increase the number of lenses.
And that’s exactly the principle of the most commonly used compound microscope:
a magnifying optical system containing several lenses, a light source and a
mechanical mount to hold the object at a defined position in front of the objective.
Compound microscopes allow for a much higher magnification and create the
image in two steps: The first lens close to the object is called the objective. The objective
creates an enlarged image of the object. This image is further enlarged by the
second lens, the eyepiece, which acts like a simple magnifying glass and magnifies
the image of the first lens. To keep away distracting light from the two lenses and
to keep their spacing in an exact position, both are mounted in a tube. To further
improve the resulting image and correct chromatic and geometric image aberration,
objectives and eyepieces may consist of multiple lenses of different glass types.
Compound microscopes are built of different components such as stand, stage
and objectives, to name just a few. The components are modular and can be exchanged
between different microscopes. Which components are used in each
case to assemble the microscope depends on the application. Depending on where
the objectives are positioned in relation to the sample, you can classify them as
upright and inverted microscopes.
The Location of the Sample Defines
the Microscope Type
Upright microscopes were the first microscopes invented and are still the classic
form of a compound microscope. Here, the sample is positioned below the objectives.
Simply said, you look down to see your sample. Upright microscopes are
usually used to study fixed samples, slides or coverslips.
On inverted microscopes, the sample is positioned above the objectives. Basically,
you look up to see the sample. Inverted microscopes are the instruments of
choice for work with cell cultures in live cell imaging. Biological samples in a fluid
tend to sink in the fluid, so you need to look from the bottom up through the petri