5 Important Types of Microscopes

This article throws light upon the five important types of microscopes. The types are: 1. Simple Microscope 2. Compound Microscope 3. Monocular Compound Microscope and Its Parts 4. Phase-Contrast Microscope 5. Electron Microscope.

Type # 1. Simple Microscope:

This is a simple device similar to the convex lens used for viewing a small object that can be seen just by bringing it close to the eye more clearly. For this purpose the microscope lens should be placed at an appropriate distance between eye and the object so that the image of the object formed by the lens is brought into focus by the eye.

However, when objects are too small the image formed by the lens may again become blurred as it is not focused by the eye.

Type # 2. Compound Microscope:

The shortcoming in the simple microscope is overcome by enlarging the image of the object by the objective lens in the first instance and then bringing the image closer to the eye with an eye piece (ocular) lens as in simple microscope or convex lens.

The magnifications of eye piece and objectives vary depending upon the make and the purpose for which it is used. The usual magnifications used in compound microscope for eye piece is 10 × whereas for objective lens there are as many as three or four.

When three are used, they are low power 10 ×, high (dry), 43 × and oil-immersion, 97 ×. The magnification of the object as observed by the viewer is approximately, magnification of objective lens × magnification of ocular lens. The magnifications of the object by the three objectives, therefore, would be 10 × 10 (100), 10 × 43 (430) and 10 × 97 (970) for low, high and oil-immersion lenses respectively.

When an object is too minute and in close proximity to the eye the most powerful compound microscope can at best magnify it but not increase its clarity so as to be seen distinctly. To overcome this difficulty the resolving powers of the lenses must be increased. It is the power of the lenses to show distinctly points in the object which appear to be overlapping when observed with the aid of the usual compound microscope.

The resolving power of a lens may be expressed by the equation:

Resolving power =Wavelength/Numerical aperture

Numerical aperture for lenses with short focal length

= n (refractive index) × diameter of lens apercu/Focal length of lens

In a compound microscope with condenser and objective of similar numerical aperture

The resolving power = Wavelength/2 × numerical aperture

Under oil immersion, oil that has same refractive index as glass is used to fill the gap between object and objective lens. The smallest object than can be seen distinctly under oil immersion lens of ordinary compound microscope with blue light of 0.47 µ wavelength is that with 0.2 µ diameter.

If the light with shorter wave length is used, say ½ that of blue light the object with diameter as small as 0.1 µ should be seen distinctly. In practice, eye is not sensitive enough to use light with short wavelengths so a photographic reproduction of the image could be used for the purpose.

Type # 3. Monocular Compound Microscope:

In biology, binocular microscopes are also used.

The essential parts of the usually used compound monocular microscope by students are the following:

I. Lenses:

1. The eye, piece with different magnifications (5-20 times). It has field lens toward the object and eye-lens close to the observer’s eye.

2. Objectives, generally with three different magnifications viz. low (10 ×), high dry (43 ×) and oil immersion (97 ×). The focal-length of these is 16 mm, 4 mm and 1.6 mm respectively. These objectives are mounted on a revolving nose-piece for convenience.

II. Adjustment of Objective Lens:

In some microscopes course and fine adjustment buttons are both provided for the body in order to lower or raise the objectives for rendering image clear. This is done by rotation of the buttons. The course adjustment is meant to bring the object into vision whereas the fine adjustment is used for observing finer details.

III. Stage:

The slide or object to be observed is placed on this. It may have only clips to keep the object in desired position or a mechanical stage for up and down and side to side movement of the fixed object. In some microscopes the stage may be raised or lowered with coarse and fine adjustments for focusing the object.

IV. Mirror:

This is of two planes—one concave and the other plane. When natural light is available the plane mirror may be used for reflection of light because concave mirror would form window images. However, with artificial illumination for higher magnifications concave mirror is necessary whereas for lower, plane mirror may be used.

V. Sub-Stage Diaphragm:

This is meant to control the amount of light permitted to the object through the condenser.

VI. Sub-Stage Condenser:

The ordinary mirror for reflecting needed light into the aperture of the objective cannot provide adequate light when objective magnification exceeds 10 ×. The illuminating cone of light of sufficient angle to fill the aperture of objectives with magnifications higher than 10 × is therefore rendered possible by use of lenses called Abbe condensers. There are used with plane mirrors.

Dark Field Illumination:

When a beam of light passes through a dark room shining dust particles too small to be seen under well-lit condition of room can be observed. This is known as the Tyndall effect. This principle is used in a microscope so that objects too small to be visible by direct illumination may be observed, counted and also their movements followed.

Further, the outlines of larger object which are less distinct with light illumination are rendered very clear. The actual shape and size may, however, be not possible to determine. The dark field illumination is made possible just by fitting in a special condenser that focuses a hollow cone of light into stage.

The rays which diverge from the stage are far too oblique to enter the objective with the result that a clear slide on a stage will appear black. Scatter light should be on the slide, part of the light enters the objective lens making the objects bright with dark background. Small organisms which are not clearly seen with transmitted light Treponema, zoospores etc. are very brightly illuminated under this method.

Type # 4. Phase-Contrast Microscope:

Unstained objects which have refractive index slightly different from their background are not clearly visible. However, the difference can be exaggerated so as to make contrast possible by modifying part of the light passing through the light microscope.

This is accomplished by introducing a de-fraction plate or coating within objective lens and annular diaphragm below the sub-stage condenser. The diaphragm controls the illumination on the de-fraction plate of the objective, which selectively modifies the light from the object in order to yield an image of greater contrast.

Type # 5. Electron Microscope:

Electrons can be made to deflect from their course by magnetic fields more or less like light by lenses. Electrons moving at a high velocity have wavelengths of 5 × 10^-6µ at 60 kV potential. When such electrons are used in a microscope as substitute for ordinary light the microscope has very high resolving power (10,000 times that of light microscope) in as much as the wavelengths of moving electrons are extremely short.

The objects which are focused cannot be directly seen, for our eyes are not adequately sensitive. Therefore, photographs are taken of only the electronic images which are equivalent to shadows of the objects on an X-ray plate.

The source of electrons is heated tungsten filament and the shaped electric magnetic fields form substitute to the glass lenses of the light microscope. The stream of electrons are directed on the object which intercepting the stream casts a shadow that is photographed.