DIATOMS IN DARK-FIELD
by Roland Mortimer,
Rio de Janeiro, Brazil
Part I and Part II of this series was presented in the February and
March 2000 issues of Micscape.
In the previous two articles on this subject I often had to remove, electronically, colour aberrations created by the dark-field condenser and the silica of the diatom frustules themselves. This time I decided to leave them to show how 'false' colours can be produced by the extremely versatile Leitz Heine condenser which gives alternating bright/phase/dark-field illumination enabling one to continuously observe the same object under the various modes of light transmission without disturbing the set-up.
The mirror element in the condenser is simply racked up or down to obtain the above types of effect and other interesting intermediate effects too. As I've said before, I study diatoms here alone, I know of nobody who studies diatoms specifically here and even one of Brazil's most famous research foundations told me there was little interest in this field here in Brazil. So, with limited literature and help only through friends via e-mail I undoubtedly make some errors in identification of species, so any help in the form of criticism (constructive) or even the odd scanned page from a book would be gratefully accepted.
All the diatoms shown here and in the previous articles were all found in a very small sample of seawater from an island near Ibicui on Rio's coast. The larger seaweeds were placed in a small plastic bag with seawater and shaken well to loosen the diatoms from the weed, then a small hole was made in one corner of the bag and the water allowed to run into a small bottle. (Klaus Kemp's tip on collecting a variation of diatom species from a small sample of seaweed). For the majority of the images the magnification used was 750X.
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Image gallery by the author.
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