Читать книгу Cell Biology - Stephen R. Bolsover - Страница 24

Today's light microscopes, such as those one would find in a school laboratory, consist of a light source, which may be the sun or an artificial light, plus three glass lenses: a condenser lens to focus light on the specimen, an objective lens to form the magnified image, and a projector lens, usually called the eyepiece, to convey the magnified image to the eye (Figures 1.8 and 1.9). Since the image is formed by light passing through the specimen, this is a transmission light microscope. Depending on the focal length of the various lenses and their arrangement, a given magnification is achieved. In bright‐field microscopy, the image that reaches the eye consists of the colors of white light minus those absorbed by the cell. Most living cells have little color and are therefore largely transparent to light. This problem can be overcome by cytochemistry, the use of colored stains to selectively highlight particular structures and organelles. However, many of these compounds are highly toxic and to be effective they often require that the cell or tissue is subjected to a series of harsh chemical treatments. A different approach, and one that can be applied to living cells, is the use of phase contrast microscopy. This relies on the fact that light travels at different speeds through regions of the cell that differ in composition. The phase contrast microscope converts these differences in refractive index into differences in contrast, and considerably more detail is revealed (Figure 1.10). Transmitted light microscopes can distinguish objects as small as about half the wavelength of the light used, so about 250 nm (nm, 1 nm = 1/1000 μm). They can therefore be used to visualize the smallest cells and the major intracellular structures and organelles (Figure 1.11a).

Figure 1.8. Basic design of a light microscope.

Figure 1.9. A simple upright light microscope.