The Small World of Freshwater Gastrotrichs
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Double Coverslip

Gastrotrichs are animals in which the upper and lower sides differ greatly. It is often necessary to observe both the ventral and dorsal sides closely. When observing an animal with a coverglass compressorium, usually only one side can be observed accurately. The other side is only accessible when focusing through the animal. However, this results in great optical quality losses - especially with high-resolution lenses. One can then only try to rotate the animal by moving the cover glass. Unfortunately, this method only has a chance of success with round objects and usually ends lethally with gastrotrichs… 😢

Double Coverslip
diagonal double cover glass vaseline bridge in yellow
To allow bilateral observation, one can enclose the object between two coverslips and turn the entire coverslip stack on the slide if needed. To do this, use two different sized coverslips. The larger coverslip (e.g., 20x20 mm) is pinned to a slide with a small drop of distilled water. The coverslip should be oriented diagonally so that it can be easily grasped at the opposite corners if necessary. A second, smaller coverslip (18x18 mm) is fitted with a vaseline ridge on two edges (as described in the article coverslip compressor and pressed onto the lower coverslip with the sample in a controlled manner, thus holding the object between the two coverslips.

The object can now be observed with the usual optical quality on the top side. Once observation from this side is complete, a drop of immersion oil is placed on the coverslip (if not already done) and the coverslip stack is detached from the slide at the protruding corners. The stack is rotated and placed with the oiled side on the slide. Now the observation can be made from the now upper, former bottom side. The use of different coverslip sizes ensures that only one coverslip is touched when the coverslip stack is rotated and that the coverslips do not shift against each other.