Microscope

                                                Microscope and its Maintenance

One of the tools essential for studying living organisms that are too small to be seen with the naked eye is the microscope.  

Depending upon the lens system, the microscope is of two kinds: 

(i) Simple microscope

(ii) Compound microscope

A simple microscope consists of a single lens system while a compound microscope consists of two or more lens system. Depending upon the source of illumination, the microscope can be classified as:

a) Light microscope, where the specimen is illuminated by visible light or ultraviolet rays. They include the bright field, dark field, ultraviolet, phase contrast and fluorescent microscopes. 

b) Electron microscope, where the image is formed on a fluorescent screen by electron beams instead of light rays and focused by magnets instead of lenses.

Aim

To study about the principle and parts of microscope used for observation of microorganisms and learn about its maintenance.

Principle

Magnification

Theoretically, the size of an image can be increased by additional lenses. The magnification, the process of enlarging the image of an object of a microscope depends on the type of the objective lens used with the ocular.

Total magnification = Magnification of objective x Magnification of Ocular

Resolving Power

The resolving power is the ability to distinguish two adjacent objects as separate and distinct image rather than as a single blurred image. 

A microscope’s resolving power is dependent on the wavelength of the beam used for illumination and on the optical quality of the lenses. Shorter wavelength give better resolution. Resolving power is determined by the formula

Resolving Power (RP) =λ/2NA

where, λ is the wavelength, NA is the numerical aperture

Numerical Aperture

The resolving powder can be increased by decreasing the numerical aperture (NA). Numerical Aperture can be defined as the function of the diameter of the objective lens in relation to its focal length. 

NA is expressed as, Numerical Aperture (NA) = n Sinϴ

where n = refractive index and ϴ = angular aperture

Limits of Resolution

It is defined as the shortest distance between two objects when they can be distinguished as two separate entities: d = λ

                    2 NA

where d = resolution, λ = wavelength of light and NA = numerical aperture

Parts of the Microscope

The bright field compound microscope (microscope that uses a direct light source for illumination) is a type of microscope, which is most commonly used for general laboratory observations. It consists of a series of two lens system between the eye and the object and a direct light source (sun or mirror or lamp). Light shines directly on the specimen and dark objects in a bright field is observable. 

Eye piece

It is a lens where final image of the object is viewed. 

Body Tube

It is roughly 160 mm in length which helps in calculating the focal length of microscope. Focal length is the distance between object and objective. 

Slide adjuster

There are two types of slide adjusters which helps in viewing the slide directly (vertical and horizontal), and with which the different fields of the object in the slide can be viewed.

Condenser

It is used to condense the light towards the object

Iris Diaphragm

It is used to cut light towards the object. Iris diaphragm should be completely open when image is viewed under oil immersion objective, partially for high power objective or completely closed or slightly opened for low power objective.

Reflective Mirror

There are two types of mirrors, plain mirror and concave mirror. Plain mirrors are used for natural source of light eg., sunlight. Concave mirror is used for artificial light eg., tube light. 

Filter

It is used to reduce the intensity of light eg., Blue filter

Oils to be used for routine use of Microscope

a) Cedar wood Oil

b) Paraffin Oil

c) Immersion Oil

MAINTENANCE OF MICROSCOPE

1) Clean the lenses of microscope with lens paper before using it. Remove oily substance from glass parts of the microscope by wiping with a lens paper moistened with Xylol. But Xylol should be removed immediately with lens paper dipped in 95 % alcohol and the lens is later on wiped dry with a fresh lens paper. 

2) Always focus the slide with the low power objective first, slowly and carefully. Then, focus the appropriate objective and visualize as per the protocol. 

3) When using high power objective (100 X), swing it partially out of way and place a drop of immersion oil on the slide and the bring the objective into position. 

4) Before putting the microscope to its cabinet, after completion of the exercise, move the nosepiece to bring a low power objective into position. Clean the oil from immersion lens using special lens paper and also clean off the slide with a tissue paper. 

                         

 

                             Microscope