Darkfield, Rheinberg and Polarization
Darkfield and Rheinberg Illumination
The condenser in a brightfield microscope illuminates the specimen with a solid cone of light. There are some simple additions that can be made to even inexpensive microscopes that dramatically enhance the image by manipulating the way light hits the specimen. These methods are best accomplished using low power objectives (4x to 10x)
Darkfield: By inserting an opaque disk (called a "stop") in the light path so that only a hollow cone of light illuminates the subject, very fine details can be resolved. The disk may be a coin placed below the condenser (size is dependent on the microscope objective used). The illumination is oblique and enters the microscope objective ONLY if refracted by the object, the background remains dark.
Rheinberg: If the central stop is made of a colored translucent material, the image background becomes the color of that material. By adding other colors to the outer cone, different objects refract those different colors. The colored filter materials are readily available from craft and art stores but can produce impressive effects as illustrated with this aquatic rotifer.
The lenses from a pair of polarizing sunglasses can add some of the most astounding effects to a simple microscope. Place one lens ("polarizer") below the condenser and fix the other lens ("analyzer") to the microscope ocular (eyepiece). Looking through the ocular and without a specimen on the stage, turn the polarizer until the field of view becomes as dark as possible ("extinction"). Then place some birefringent object on the stage - it can be a chemical crystal, cellophane wrapper, mica, or even small creatures (the muscles are birefringent).
This small circle (left) shows a crystal between a polarizer and analyzer. The range of colors was recorded by rotating the polarizer, but you can also change the colors by rotating the crystal. Polarization is used on some of the most expensive research microscopes to see the spindle fibers in dividing cells or the contractile proteins in muscle cells.
A gallery of chemical landscapes photographed using polarization microscopy is posted elsewhere on this website at Crystals alive!
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