Nothing was known about the cell and it even existed before the invention of the microscope. The study of cell biology has been inseparable from the evolution of the microscope. The 19th century was an era in which the cell was investigated extensively with the microscope.
An unaided human eye’s resolution limit is about 100 microns under optimal viewing conditions. Thus, a man cannot see those objects separately which are separated by a distance of fewer than 100 microns and the object itself smaller than 100 microns.
The limitation of the human eye helped in the evolution of various magnifying investments. First came the simple magnifying glasses then compound microscopes were devised.
Recently developed electron microscopes gradually extended the resolving power to 1,00,000 times greater than the light microscope.
Microscopes are essential in studying cells because they possess magnification characteristics and resolving power between two objects lying close together.
The lesser the distance that can be measured between two points in a microscope field, the greater its resolving power.
The capacity of the microscope both to magnify and resolve depends on the nature of light as a source of illumination cannot resolve objects less than one-half the wavelength of light.
The average wavelength of white light is about 0.55 microns. Therefore the microscope using whole light cannot resolve objects less than about 0.2 microns.
A compound microscope is used to produce a considerable magnification of small objects. The essential parts of the instruments consist of two convex lens systems, placed co-axially within two sliding tubes at a distance from each other.
The first lens which is turned towards the objects is called the objective lens. It is of a very short focal length and has a small aperture.
The other convex lens through which observation is made is called the ocular lens of the eyepiece. It has a larger focal length and a wider aperture than the objective lens.
The objective lens produces an initial image of the object. The ocular lens magnifies the initial image and produces the final image.
To see objects smaller than 0.2 microns electron microscope is used where a beam of the electron is used as an illumination source. They have a wavelength of about 0.50 Angstroms.
Thus, the resolving power of the electron microscope can be one-half of the 0.50 Angstroms. The electron microscope was developed in the eye of World War Second.
A considerable increase in resolving power has been made possible by their development. The electron microscope since the time of its design has proved a powerful tool of great value for both science and industry.
The major difference between the electron microscope and the light microscope and the light microscope is the physical design of the two instruments.
The electrons can travel to a good distance in a vacuum so the entire electron microscope unit is enclosed in a vacuum-tight column.
A metal filament heated in a vacuum emits a narrow beam of electrons. The electron beam is collected and focussed upon the specimen by the electron-magnetic condenser lens and electrons are collected by the electron-magnetic objective lens after passing through the object.
The objective lens enlarges the image of objects. The electromagnetic projector of the “eyepiece” lens further magnifies the image of the object and projects it on a fluorescent viewing screen or on the photographic plate.
Phase contrast microscope
It is used for the study of living cells that are, in general, transparent to light. The principle on which this microscope is based is that the light passing through an object undergoes a retardation or phase change which normally is not detected.
In this instrument, however, the phase difference is advanced or retarded to one-fourth of the wavelength and the small variations in phase produced by the various structures are thereby made visible.
The ultraviolet microscope looks like a compound microscope. It differs from light microscopes in its lenses. The lenses are made up of quartz to permit the transmission of ultraviolet light.
The ultraviolet microscope is very much helpful in the study of chromosomes and nucleic acids (DNA and RNA) because they absorb more short ultraviolet than the cytoplasm.
The polarizing microscope is similar in principle and constitution to a compound microscope. The polarising microscope can detect regions in cells where constituents are disposed of in a highly ordered array. This is done with the help of a prism. The prism transmits plane-polarized light instead of ordinary light.
The dark field microscope is an ordinary microscope with a special condenser. This condenser stops the beam of light from the center of the field and the objects are illuminated only by an oblique beam of light.
Due to the oblique beam of light, this microscope is suitable for the study of outlines of cells, nucleus, oil droplets, vacuoles, mitochondria, etc.