Biological polarized light microscopy
by John S. Wojtowicz, US
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The colorful views of anisotropic crystals under crossed polars are a delight to many microscopists. Less well known is the fact that some types of muscle tissue exhibit similar properties. These can be well seen in many freshwater microcrustacea such as the Daphnia pictured here (right, brightfield).
The crossed polar images (right) very clearly shows the antennal levator and adductor muscles compared with the brightfield image. Additional colorful effects (below) can be obtained by the addition of a wave retarding plate located between the polarizing filters. The professional ones have very precisely defined properties. The color effects were obtained here by simply holding the lid of a plastic petri dish above the polarizing filter on the illuminator. Scraps of cellophane from various household sources will work well if of an appropriate thickness.
Unlike crystals, critters can be difficult to keep still long enough for the exposure times required for crossed polars. The difference in exposure required for a given subject in both brightfield and crossed polars can be well beyond the range of many flash units. These photographs were obtained by using a microscope slide with two cover slips cemented about 5mm apart. The subject was placed in a drop of water in the gap, posed and a cover slip placed on top. The slide was then very gently heated until the subject was killed and photographed immediately. I find this system both cheaper and better than the commercial cavity slides because the depth of the fluid is even, unlike a hanging drop.
Comments to the author John S. Wojtowicz welcomed.
Published in the September 1999 edition of Micscape Magazine.
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