Both the polarizer and the analyzer can be rotated; the difference between their angles of rotation is used to determine the degree by which a structure affects the beam of polarized light. The ability of a crystal or paracrystalline array to rotate the plane of polarized light is called birefringence (double refraction). Striated muscle and the crystalloid inclusions in the testicular interstitial cells (Leydig cells), among other common structures, exhibit birefringence.
Two kinds of EMs can provide morphologic and analytic data on cells and tissues: the transmission electron microscope and the scanning electron microscope. The primary improvement in the EM versus the light microscope is that the wavelength of the EM beam is approximately 1/2,000 that of the light microscope beam, thereby increasing resolution by a factor of 103.
The TEM uses the interaction of a beam of electrons with a specimen to produce an image.
The optics of the TEM are, in principle, similar to those of the light microscope (see Fig. 1.9), except that the TEM uses a beam of electrons rather than a beam of light. The principle of the microscope is as follows:  An electron source (cathode, electron gun), such as a heated tungsten filament, emits electrons.  The electrons are attracted toward an anode.  An electrical difference between the cathode cover and the anode imparts an accelerating voltage of between 20,000 and 200,000 volts to the electrons, creating the electron beam.  The beam then passes through a series of electromag netic lenses that serve the same function as the glass lenses of a light microscope.
The condenser lens shapes and changes the diameter of the electron beam that reaches the specimen plane. The beam that has passed through the specimen is then focused and magnified by an objective lens and then further magnified by one or more projector lenses. The final image is viewed on a phosphor-coated ﬂuorescent screen or captured on a  photographic plate. Portions of the specimen through which electrons have passed appear bright; dark portions of the specimen have absorbed or scattered electrons because of their inherent density or because of heavy metals added during specimen preparation. Often, an electron detector with a light-sensitive sensor such as a charge-coupled device (CCD) are placed above or below the viewing screen to observe the image in real time on a monitor. This allows for uncomplicated procedures of archiving images or videos in digital format on computers.
Specimen preparation for transmission electron microscopy is similar to that for light microscopy except that it requires finer methods.
The principles used in the preparation of sections for viewing with the TEM are essentially the same as those used in light microscopy, with the added constraint that at every step one must work with specimens three to four orders of magnitude smaller or thinner than those used for light microscopy. The TEM, which has an electron beam wavelength of approximately 0.1 nm, has a theoretical resolution of 0.05 nm.
Because of the exceptional resolution of the TEM, the quality of fixation—that is, the degree of preservation of sub-cellular structure—must be the best achievable.
Routine preparation of specimens for transmission electron microscopy begins with glutaraldehyde fixation followed by a buffer rinse and fixation with osmium tetroxide.
Glutaraldehyde, a dialdehyde, preserves protein constituents by cross-linking them; the osmium tetroxide reacts with lipids, particularly phospholipids. The osmium also imparts electron density to cell and tissue structures because it is a heavy metal, thus enhancing subsequent image formation in the TEM.
Ideally, tissues should be perfused with buffered glutaraldehyde before excision from the animal. More commonly, tissue pieces no more than 1 mm3 are fixed for the TEM (compared with light microscope specimens, which may be measured in centimeters). The dehydration process is identical to that used in light microscopy, and the tissue is infiltrated with a monomeric resin, usually an epoxy resin, that is subsequently polymerized.