Some of the major types of light microscopes are as follows: A. Dark-field Microscope (=Ultra-microscope) B. Phase-Contrast microscope C.
Differential-interference-contrast microscope: (= Nomarski interference microscope) D. Ultraviolet Microscope E. Fluorescence microscope F. Confocal microscope.
A. Dark-field Microscope (=Ultra-microscope):
It was invented by Zsigmondy (1905). In dark field microscope, a special condenser lens is used with an opaque disc at the centre, so that direct rays don’t enter the objective lens. Only light scattered by the specimen enter the objective lens to form a bright image against dark background. Dark field microscope does not have a good resolution. It is used in microbiology and in autoradiography.
B. Phase-Contrast microscope:
It is a modified light microscope developed by Frederick Zernicke (1932). In this microscope an annular diaphragm issued below the condenser lens and a transparency phase plate above the objective. This arrangement in microscope causes differences in light intensity while passing through the specimen, so that denser parts (organelles) alter the path of light m ore than the less dense cytoplasmic region. The image formed, therefore, has a varying contrast for different regions. It is used to study the unstained living cells and for viewing the changes during cell division, pseudopodia formation, modocytosis, exocytosis etc.
C. Differential-interference-contrast microscope: (= Nomarski interference microscope)
It gives a better image than that produced by phase contrast microscope. It is used to visualize living cells and for quantitative studies of various macromolecules of the cells. It also measures dry weight, thickness and water content of the specimen.
D. Ultraviolet Microscope:
Casperson (1938) invented ultraviolet microscope where the illuminating source is ultraviolet rays from mercury or iodine-Quartz lamp. Image is captured on photographic film.
E. Fluorescence microscope:
It is a modification of ultraviolet microscope. Fluorescent dyes (e.g. fluorescein, rhodamine), absorb UV light and emit as longer wavelength light, are used to stain specific macromolecules of cell. The florescence microscope makes the use of two filters, one filter permits only short wavelength light to pass through the specimen for observation. Another one is used to block light emitted by fluorescent dye. It is used to study cyto-chemical reactions within living cells.
F. Confocal microscope:
In this case a laser beam is focused to scan the: specimen, which produce a clear image of one plane of the specimen, while other planes of the specimen are excluded and don’t blur the image.