Microscopes are indispensable instruments in modern laboratories, no research would be possible without it in any institute or a diagnostic center.
The human eye is a natural optical system, characterized by specific resolution, ie the smallest distance between elements of the observed object (perceived as points or lines) where they can still be different from one another. For the normal eye away from the object on Vol. the best distance vision (D = 250 mm), the average normal resolution is 0,176 mm. The size of microorganisms, the majority of plant and animal cells, small crystals, details of the microstructure of metals and alloys, etc., are much less than this value.
Until the mid XX century they used only with the visible optical radiation in the range of 400-700 nm, as well as with short-UV (fluorescent microscope). Optical microscopes can not give the resolving power of less than a half-cycle of the reference wave radiation (wavelength range of 0,2-0,7 mm, or 200-700 nm). Thus, an optical microscope is meant to discern the structure with the distance between points to about 0.20 microns, so the maximum increase, which could be achieved was ~ 2000 times.