The last step in the histological process is visualizing the result of the technique performed. The resolution power of the human eye is 0.2 mm (resolution power: capacity for resolving two closely located points), and a typical eukaryote cell is commonly between 10 and 50 µm in size (µm. 1 µm=10-6 mm). Moreover, if the ultrastructure of the cell is going to be studied, we need to visualize cell membranes, which are about 7 nm thick (nm; 1 nm = 10 -6 µm). Hence, we need the help of devices to magnify and visualize very small structures like cells and cell compartments. These devices are known as microscopes.
There are two types of microscopes: light and electron microscopes
Light microscopes, or optical microscopes, use visible light and glass lenses to magnify samples up to 1000 times, with a resolution power of 0.2 µm. This is the highest resolution that visible light is able to provide because of its wavelength. Light microscopes are common for general observation of tissues and cells, and can be found in all histology laboratories.
Electron microscopes are based on the very short wavelength of electrons, and get a resolution power of 1 nm. They are used for studying the ultrastructure of cells and tissues, that is, the subcellular level organelles, membranes and molecular complexes (for example, ribosomes). There are two types of electron microscopes: transmission and scanning. The ultrastructure of tissues in ultrathin sections can be studied with transmission electron microscopes. Scanning electron microscopes are intended for the observation of surfaces.
The capabilities of microscopes, particularly optical microscopes, can be extended by adding some devices. For example, optical microscopes can be modified for observing fluorescent molecules, or to study unstained tissues. Each of these particular modification are a type of microscopy. Thus, there is fluorescence microscopy, phase contrast microscopy, dark field microscopy, etcetera.
The current research in cell biology is based in microscopes. The improvement of microscopes over the years has lead the new discoveries about cells, and complex microscopy techniques have widen the acknowledge of cell features. The cell theory is an outcome of research by using microscopes.