Trace the evolution of the microscope in the study of cell structure

The structural components of the cell are best seen with a microscope. Early microscopes utilized light that passed through extremely thin sections of cellular material. These sections were first fixed with substances such as formaldehyde that stabilized the components of the cell. Appropriate strains were then applied that could render visible specific organelles. The cells were, however, killed by the preparation process. With the development of the interference and phase-contrast microscopes, living material could be studied without the intervention of fixing and staining. These microscopes make use of the fact that light traveling through materials of different densities can be altered by special devices that tend to emphasize the contrast between adjacent structures. Polarizing microscopes utilize a beam of polarized light (wave movement in a single plane rather than in all directions) to distinguish those areas of a cell in which there is a regular alignment of constituent parts.

Perhaps most fruitful in the study of cellular organization is the electron microscope, a device employing a beam of electrons rather than light.

The electron microscope requires a vacuum chamber, since electron beams are dissipated by air. A magnified image from the electron beam is visualized on a fluorescent screen and may also be permanently recorded on a photographic plate. The preparation of the specimen for electron microscopy is more tedious and requires even greater care than that for light microscopy.

The utility of an optical instrument such as the microscope can be characterized by its resolution or resolving power. The resolving power represents a measure of the minimum distance by which two closely spaced objects can be clearly elucidated and identified. The resolving power of a light microscope is 0.25mm. By comparison, the resolving power of the human eye is 0.2 mm. The use of electrons permits a resolution (separation of neighboring particles) that is more than 1000 times better than that of the light microscope. Thus, two points that are only 0.2 nm apart can be seen distinctly with the electron microscope.