Histology Microscope


Compound Histology Microscope

The compound microscope is the main instrument used to study histology. A microscope is an expensive instrument. It must be handled carefully and used properly at all times. Learning the proper techniques for using a microscope to view histology slides will keep the microscope in proper working order, avoid physical headaches, avoid eyestrain and enable optimal visualization of the histology slide.

The compound microscope utilizes a system of precision ground lenses and aligned illumination to give a sharply magnified image of the histology slide. A microscope is considered compound because of the dual lens system. The magnification obtained by compound microscopes varies from about 10X to 2000X.

Optical System on the Histology Microscope

The optical system of the typical compound microscope consists of the eyepiece (also called the ocular) and the objective lenses.

Eyepiece / Ocular
The lens on a microscope that is nearest your eye is called the eyepiece or ocular. If a microscope has two eyepieces, it is called a binocular microscope. If a microscope has only one eyepiece, it is called a monocular microscope. Most eyepieces produce a magnification of 10X, however some are 15X. The distance between the two eyepieces can be adjusted to accommodate various interpupillary distances. The eyepiece tube holds the eyepiece. The tube is the part of the microscope that connects the eyepiece to the objective lenses.

Objective Lens
The objective lenses are the lenses on a compound microscope which are closest to the histology slide. The role of the objective lenses is to magnify the image of the histology slide. Usually there are three to four objective lenses on a microscope. Objective lenses are available in different powers: usually 4X, 10X, 40X, and 100X. The objective lenses vary in length: the longer the objective lens, the greater the magnifying power.

The objective lenses are arranged on a revolving nosepiece (also called a turret). The revolving nosepiece holds several objective lenses. The lenses then can be easily rotated into position for the desired magnification.

The 4X objective is used to get an overview of the histology slide. This magnification is used to spot regions which require more detailed observation of the histology. The 10X objective is the most useful for actual tissue identification of the histology slide. The 40X objective is also used for tissue identification and to see the finer details and study tissue organization on the histology slide. The 100X lens requires the use of immersion oil. A drop of oil goes between the objective and the histology slide. This objective is used primarily to examine the histology of blood cells.

The total magnification of the histology slide is the product of the magnification of the eyepiece and the magnification of the objective. For example, with a 40X objective, the total magnification is 400X (10X multiplied by 40X). With the objectives of 4X, 10X, 40X and 100X and a standard eyepiece, the possible magnifications are: 40X, 100X, 400X, and 1000X.

Illumination System of the Histology Microscope

The illumination systems used on compound microscopes vary. The most typical system consists of three major parts. First, a consolable lamp, or illuminator, mounted into the base. The second part is the iris diaphragm. The third part is the condenser. The role of the illumination system is to make a cylindrical light beam.

Consolable Lamp / Illuminator
A microscope needs a light source: a consolable lamp or illuminator mounted into the base. There is a power switch to turn on and off the light source. There is some sort of illumination control to adjust the brightness, such as a sliding switch.

Some microscopes use a mirror to reflect light from an outside light source, rather than a light.

Iris Diaphragm
There may be a diaphragm mounted above this light source, underneath the stage. It is a rotating disk with holes of various sizes. This controls the intensity and diameter of the light cone. This diaphragm functions to limit and direct light into the next components.

The iris diaphragm opens and closes to allow a column of light through it. The iris diaphragm controls the diameter of the light beam, similar to the iris of the eye. When a column of light is the same diameter as the objective lens, there will be much less light scatter. The iris is used to adjust contrast.

The condenser is a lens or system of lenses on the lower part of the stage. After light passes through the iris diaphragm, the condenser converges the light rays. Without the condenser, the light would go every direction. The function of the condenser is to focus the light beam onto the histology slide. There is a a condenser focusing knob for adjustment.

Fine Focus and Course Focus on the Histology Microscope

The fine focus knob and course focus knob are used for focusing the image of the histology slide. The focus controls move the specimen stage up or down in order to bring the histology specimen into focus. Coarse focusing is used initially with the lowest objective (4X). After this, only fine focusing is used. Fine focus is used after the image has been brought into focus at a lower power.

A microscope is considered parfocal if practically no change in focus has to be made
when the magnification of the objective is changed.

Some microscopes have a focus stop to prevent the slide from bumping into the
objective lens.

Mechanical Stage and Stage Clips on the Histology Microscope

Beneath the objective lenses sits the flat mechanical stage or specimen stage. The
histology slide is placed here. The stage clips (or slide holder) holds the histology slide
in place. On the side of the microscope are two knobs (slide holder travel controls).
These move the histology slide around. One knob moves the histology slide to the left and right, the other knob moves the histology slide to the front and back.

Arm of the Histology Microscope

The arm is the part of the microscope that connects the body tube to the base.