The Compound Microscope

Basically, compound binocular microscope is made up of mechanical system, that will support the compound binocular microscope, and an optical system. The optical system illuminates the object under the examination and passes the light through a lot of lenses to form a picture of the specimen on the retina of the eye.

The mechanical system is composed of six parts:
Base- the support in which the instrument hold on to.

Arm is the one that we use to hold the compound binocular microscope. This part of the compound binocular microscope is very important to move the compound microscope from one place to another.

Stage is the horizontal plate where the specimen is placed for observation. The specimens are usually placed on microscope glass slides that are held firmly in place on the stage by means of microscope stage clips.

Body tube is a cylindrical hollow tube on which the objective and eyepiece lenses are placed on the opposite ends. Coarse adjustment is the knob that focuses the compound binocular microscope lenses on the specimen by turning the body tube upside down.

Fine adjustment the movement affected with this knob is similar to those of the coarse adjustment but is of a much smaller magnitude.

The optical system is made up of four parts:
Illuminator is the most modern in compound binocular microscopes that uses artificial light supplied by a light bulb to illuminate the specimen being examined or studied. If the specimen is transparent, the light is directed up toward and through the specimen stage from an illuminator built into the base of the compound binocular microscope. This is known as transmitted illumination. When the object is opaque-that is, not transparent-the light source must be placed above the specimen so that in can be reflected off the specimen’s surface and into the lens system of the microscope. This type of illumination is known as vertical or reflected illumination.

Condenser is another part of the optical system of compound binocular microscope. The condenser gathers light rays from the base illuminator and concentrates them on the specimen. The simplest condenser is known as the Abbe’s condenser. It consists of two lenses held together in a metal mount. The condenser also includes an iris diaphragm that can be opened or closed to control the amount if light passing into the condenser.

Objective Lens is the third part of compound binocular microscope optical system. This is the lens located closest to the specimen. To help with the changing from one objective lens to another, several objectives are mounted on a revolving nosepiece or turret located above the specimen. Most compound binocular microscopes are parfocal, which means that when the microscope is focused with one objective in position, the other objected can be rotated into place by revolving the nosepiece while the specimens remains very nearly in correct focus.

Eyepiece or Ocular Lens is the last part of the compound binocular microscope. This is the lens closest to the eye. A compound binocular microscope has two eyepieces, some with one eyepiece is called monocular compound microscope and also the one which has three eyepieces is called trinocular compound microscope.

Each microscope lens has a number signifying its magnifying power. The image that is viewed by the microscope expert will have a total magnification that is equal to the product of the magnifying power of the objective and eyepiece lenses. For example, an eyepiece lens with the magnification of 10 times (10x) used in combination in 100 times (100x). Most forensic work will require a 10x eyepiece in combination with a 4x, 10x, 20x, or 45x objective. The respective magnification will be 40x, 100x, 200x, and 450x. Simply multiply to get proper magnification.

In addition, each objective lens is inscribed with its numerical aperture. The ability of an objective lens to resolve details into separate images instead of one blurred image is directly proportional to the numerical aperture value of the objective lens. For example an object lens of some numerical aperture separate details into separate images that are twice as closed as compared to a lens of another numerical aperture. The maximum useful magnification of a compound binocular microscope is approximately 1000 times the Numerical Aperture of the objective used. This magnification is enough to allow the eye to see the detail that can be resolved. Any effort to increase the total magnification beyond this will yield no additional detail and is referred to as empty magnification.

Although a new student of the compound binocular microscope may be tempted to immediately choose the highest magnifying power available to view a specimen, the experienced compound binocular microscope user realizes that a number of important factors must be made before the selection of magnifying power is made. The first consideration must be size of the specimen area, or the field of view, that the examiner wishes to study. As magnifying power increases, the filed of view decreases. Thus, it is best to first select a low magnification in which a good general overall view of the specimen is seen and to switch later to a higher power in which a smaller portion of the specimen can be viewed in more detail.

The depth of focus is also one of the uses of the magnifying power. After a focus has been achieved on a specimen, the depth of focus defines the thickness of that specimen. Areas lying above and below these regions will be blurred and can be viewed only when the focus is readjusted. Depth of focus decreases as magnifying power increases. Read more on this subject