- To understand the phase contrast microscope, it is first necessary to review the ordinary compound light microscope. It features a set of components that function together to transmit light from the subject being studied to the observer's eye. From the bottom up, these are the light source, the substage condenser, the mechanical stage, the glass slide holding the subject matter, the objective lens and the eyepiece lens.
- Viewing a specimen through the ordinary compound light microscope---called the bright field microscope---involves illuminating the object from below through an opening in the stage. The user selects an objective lens of the desired power by turning the nosepiece just above the microscope's stage. Peering through the microscope's eyepiece, the operator then adjusts the focus controls to bring the specimen into clear view.
- As the name suggests, the difference between the ordinary light microscope (the bright field microscope) and the phase contrast microscope concerns contrast. Living organisms or living cells tend to have low contrast, making details of their structure difficult to see. Some structures in a specimen, like the cilia of a paramecium or the flagellum of a euglena for example, may not have very much contrast compared to the surrounding medium. They would therefore not show up very well under regular bright field microscopy. Treating that specimen with certain stains can compensate for this. But the tradeoff is that this will kill the cells, preventing the user from observing details of a living cell.
- A phase contrast microscope, on the other hand, would correct for this by producing the desired contrast without killing the living cells. It seems to magically transform the view of a specimen showing very little contrast or detail in the bright field microscope into a dramatic image.
- The phase contrast microscope is essentially an ordinary light microscope with all the same basic components. The difference is that the phase contrast microscope features something called a phase plate. This plate is situated in the light path between the subject matter being viewed and the viewer's eye (actually in the objective lens housing). Additionally, a so-called phase ring or annulus is installed onto the substage condenser (the device that focuses light onto the specimen).
- The optical physics involved in the process can be a bit difficult. But suffice it to say that the phase contrast device alters the wavelengths of light that are traveling through the medium containing the specimen and the specimen itself. This wavelength alteration---produced by the combination of the phase plate and the phase ring or annulus---generates the desired contrast. Regular bright field light microscopes can be converted to phase contrast microscopes. This is done by installing the phase ring and objective lenses with the requisite phase plates manufactured into them.
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