A compound microscope is used to magnify samples that are not easily visible to the human eye. The magnification power of a microscope depends on the lens, which can be either 400X, 1000x or up to 1500X. Such high magnification is achieved by the use of twin lenses.

A compound microscope is used in schools, laboratories, and pathological setups to identify various samples of bacterial and viral diseases. The lens is extremely sensitive and can help identify various fingerprints, the presence of any external metals, etc.  

The microscope comprises two components, namely, the structural and the optical components, and the essentials of a compound microscope diagram will be discussed as we read on.

The structural and optical components of a compound microscope

Structural component

The structural components of the compound microscope are the head, arm, and base of the microscope. The head comprises all the optical parts, the arm is a connector for the base and the head, and the base is on which the whole microscope rests. The arm is also used to lift the microscope if required.

Optical component

The two optical systems in a compound microscope are the eyepiece lens and the objective lens. Other parts of the optical component are: 

• The eyepiece is through which we look into the microscope. A standard eyepiece has a magnifying power starting from 5x, 10x, and so on, up to 30x. 
• The eyepiece tube is a place to hold the objective lens in place. The objective lenses are the primary lenses on the microscope. The range of an objective lens can vary from 4x to 100x.
• The nose piece has the objective lens. The objectives are mounted such that they can be rotated to find the best objective lens to see through. The coarse and fine knobs help to improve the focus of the objective lens. 
• The stage of the microscope is the placement for the sample to be viewed. The stage clips keep the sample to be observed in place. The aperture is a space on the stage through which light reaches the stage. The light source for a microscope is the illuminator at the base of the microscope.

Magnification quotient of a compound microscope

The magnifying power is typically quoted as the power of the objective lens, such as 4X, 15X, 100X etc., multiplied by the power of the eyepiece, which is 10X. So if you have a 10X eyepiece and the objective lens has the power of 50X, then your magnification produced will be 500X. 

The equation used for depicting the magnification of a microscope is given by,

            m    = D/ fo * L/fe    

    where, D = reference seeing distance (25 cm)

L = Length of the microscope tube

            fo = Focal length of the objective lens

            fe = Focal length of the eye-piece lens

Uses of a compound microscope

Though many types of compound microscopes are used for testing biological samples, the utility of a compound microscope is not restricted to a biological laboratory. 

• Phase-contrast microscope: A phase-contrast microscope is used for checking a translucent sample by using different types of contrasting stains for the specimen. The refractive index of the specimen will cause the transparent sample to appear brighter than the dark background. It is preferable for observing unstained cells. 
• Polarising microscope: A polarised microscope uses a polariser to intervene between the light source and the specimen. The light is converted into a plane polarised light before it touches the sample. This microscope is used to examine specimens made of crystals and chemical mineralogy. 
• Metallurgy microscope: Unlike regular biological microscopes, metallurgy microscopes parts consist of a high-resolution optical system. As metallurgical microscopes are used in the industrial world, metals do not allow light to pass very easily through them. This requires a special technique to illuminate the sample with great technicality to highlight specific features of the sample. 
• Fluorescence microscope: The fluorescence microscope uses microscopes of differing wavelengths to highlight the sample. The principle used here is fluorescence rather than absorption, refraction or reflection.   

Conclusion

Compound microscopes are essential for specimen study. It has two parts, structural and optical components. The magnification of a compound microscope can be easily calculated by using the power of the eyepiece and the objective lens. Apart from biological microscopes used to study stains, and bacterial and viral cultures, there are metallurgical, fluorescence, phase-contrast and polarising microscopes for crystals and metals.