Microscope

The familiar monocular compound optical microscopes (i.e., microscopes that have a single lens as the eyepiece, or ocular, lens) are being replaced in many laboratories with binocular styles. These microscopes have a single objective lens, but two ocular ones, each in its own eyepiece. Light coming through the objective lens is split into two beams by a prism. Each eye sees the exact same image, so there is no three dimensional effect.

For a three-dimensional view, scientists use a stereoscopic binocular microscope. This instrument consists of two separate sets of objective lenses as well as two separate ocular lenses. Prisms alter the angle of light coming through each pair of lenses, so each eye sees a slightly different image.

Living or stained specimens often yield poor images when viewed in bright-field illumination. To help with this, scientists developed a phase-contrast microscope that alters the phase differences (the position in space of the waves and troughs) in light waves as they pass through the specimen. This makes some parts of the object brighter and others darker than normal, allowing for a better view of the structural details of the object. Closely related to this type is the interference microscope that superimposes one field of view over a second to improve contrast.

Another optical version of a microscope is called the darkfield microscope. This microscope has proven to be particularly useful for biological studies. The dark-field microscope uses a specialized illumination technique that capitalizes on indirect illumination to enhance contrast in specimens. An opaque disk is set in a condenser under the stage of the microscope (the solid support that the object being studied rests on). The disc is also known as the stop. The stop prevents light from shining directly on the specimen. Instead, light passes around the stop and is reflected off the condenser's walls.

All the microscopes described so far produce black and white images. To observe color in cells, scientists use polarizing microscopes. The microscope aligns the vibrations of a light wave by directing it through a specially cut prism. If two beams of polarized light are transmitted through the cell, as in a differential polarization microscope, researchers can make quantitative measurements of things such as cell depth and the quantity of certain cell constituents.