FIGURE 4-9 Optical microscopes are commonly configured for bright-field, phase-contrast, or fluorescence microscopy. (a) In a typical light microscope, the specimen is usually mounted on a transparent glass slide and positioned on the movable specimen stage. (b) In bright-field light microscopy, light from a tungsten lamp is focused on the specimen by a condenser lens below the stage; the light travels the pathway shown in yellow. (c) In phase-contrast microscopy, incident light passes through an annular diaphragm, which focuses a circular annulus (ring) of light on the specimen. Light that passes unobstructed through the specimen is focused by the objective lens onto the thicker gray ring of the phase plate, which absorbs some of the direct light and alters its phase by one-quarter of a wavelength. If a specimen refracts (bends) or diffracts the light, the phase of some light waves is altered (green lines), and the light waves pass through the clear region of the phase plate. The refracted and unrefracted light is recombined at the image plane to form the image. (d) In fluorescence microscopy, a beam of light from a mercury lamp (gray lines) is directed to the excitation filter, which allows only the correct wavelength of light to pass through (green lines). The light is then reflected off a dichroic mirror and through the objective lens, which focuses it on the specimen. The fluorescent light emitted by the specimen (red lines) passes up through the objective lens, then through the dichroic mirror, and is focused and recorded on the detector at the image plane.