A compound microscope is the most commonly used microscope. It magnifies the size of the object by a complex system of lens arrangement. Zacharias Janssen invented the first compound microscope in the year 1590, and later Galileo Galilei, the great Italian physicist came up with his self-made version.
A compound microscope has a series of two lenses;
• the objective lens close to the object to be observed and
• The ocular lens or eyepiece, through which the image is viewed by eye.
Light from a light source (mirror or electric lamp) passes through an object. The objective lens produces a magnified ‘real image’ ( first image) of the object. This image is again magnified by the eyepiece to produce a magnified ‘virtual image’ (final image), which can be seen by eye through the eyepiece.As light passes directly from the light source to the eye, the field of vision is brightly illuminated. That is why; it is a bright-field microscope.
Parts of a Compound Microscope:
The parts of a compound microscope are of two categories as given below:
• Base or Metal Stand:The whole microscope rests on this base. Mirror, if present, is fitted to it.
• Pillars: It is a pair of elevations on the base, by which the body of the microscope is held to the base
• Inclination joint: It is a movable joint, through which the body of the microscope is held to the base by the pillars. The body can be bent at this joint into any inclined position, as desired by the observer, for easier observation. In new models, the body is permanently fixed to the base in an inclined position, thus needing no pillar or joint.
• Curved Arm: It is a curved structure held by the pillars. It holds the stage, body tube, fine adjustment and coarse adjustment.
• Body Tube:
Body tube is a vertical tube that holds the eyepiece at the top and the revolving nosepiece with the objectives at the bottom. The length of the draw tube is called ‘mechanical tube length’ and is usually 140-180 mm
• Draw Tube: It is the upper, slightly narrower part of the body tube, into which the eyepiece is slipped during observation.
• Coarse Adjustment: It is a knob with rack and pinion mechanism to move the body tube up and down to bring the specimen into general focus. As rotation of the knob through a small angle moves the body tube through a long distance relative to the object, it can perform coarse adjustment
• Fine Adjustment: It is a small knob used for fine adjustment to get the final clear image.
• Stage: Stage is a horizontal platform projecting from the curved arm. It has a hole called aperture at the centre, upon which the object to be viewed is placed on a slide. Light from the light source present below the stage passes through the aperture
• Mechanical Stage (Slide Mover): Mechanical stage consists of two knobs with rack and pinion mechanism. The slide containing the object is clipped to it and moved on the stage in two dimensions by rotating the knobs, so as to focus the required portion of the object.
• Revolving Nosepiece:
It is a rotatable disc at the bottom of the body tube with three or four objectives mounted on it. Based on the required magnification, the nosepiece is rotated, so that only the objective specified for the required magnification can be conveniently selected.
These parts are involved in the passage of light through the object and magnification
The components of optical parts include the following:
• Light Source: Modern microscopes have in-built source in the base. The illuminator is connected to the mains through a regulator, which controls the brightness of the field. In old models, a mirror is used as the light source which is fixed to the base by a binnacle, through which it can be rotated, so as to converge light on the object.
• Diaphragm: An iris diaphragm is fixed below the condenser to control the amount of light entering into the condenser so that object can be properly visualized.
• Objective: Objective lens is the most important lens in a microscope. Typically there are three objectives with different magnifying powers are screwed to the revolving nosepiece.
The objectives are:
(a) Low power objective (X 10) (b) High dry objective (X 40) (c) Oil-immersion objective (X100)
• Condenser: Condenser is used to collect and focus the light from the light source onto the specimen to be viewed. It is located under the stage.
• Eyepiece: The eyepiece fits loosely into the draw tube. It magnifies the magnified real image formed by the objective to a still greatly magnified virtual image to be seen by the eye. Each microscope is provided with two types of eyepieces with different magnifying powers (X10 and X25).
Uses of a Compound Microscope
• A compound microscope is of great use in pathology labs so as to identify diseases.
• Various crime cases are detected and solved by drawing out human cells and examining them under the microscope in forensic laboratories. The presence or absence of minerals can be determined, and the presence of metals can be identified, thereby aiding in solving crimes.
• Forensic experts and scientists can also find out the country from which a particular drug has come by viewing its particles under a compound microscope.
• Students in schools and colleges are benefited by the use of a microscope for conducting their academic experiments. It is of immense help as the students can now see the bacteria and virus, which is otherwise invisible to the naked eye. Thereby they witness things which they have studied in theory.
• Plant cells are examined and the microorganisms thriving on it can be ascertained with the help of a compound microscope. Thereby, a compound microscope has proved beneficial to biologists too.