You have just returned from exploring the local pond, seashore or stream in search of exotic microscopic life. A collection of bounty filled jars litters your work table, your microscope, slides and cover slips are at the ready, now what?
At this point you could simply suck up a bit of the "sludge" from the bottom of the nearest jar, squirt a quantity onto a slide, drop on the coverslip and pop the whole thing onto the stage - bingo you're in business. However, subsequent examination may prove frustrating as organisms not only zoom across the field of view, but continually drift in and out of focus as they move up and down in the "ocean" of liquid beneath the slip. A bit of front end finesse can sometimes provide a more rewarding viewing experience. Let me explain.
First, let the jars rest on the table for a few hours before sampling to allow time for the fine suspended matter (often diatoms or desmids) to settle to the bottom. Once the micro-world in each jar has stabilized you can begin to examine the bottom "sludge".This sludge may hold a diverse community of interesting organisms. However, it may also contain small sand grains, organic debris and other "thick" material that will come between the coverslip and the slide. When this happens the film of water will be too deep for ideal viewing, too much room for the critters to roam! This residue can be removed by the following technique:
(1) Fill an eyedropper with sludge and water, and let the dropper rest in an upright position for a few minutes (heavy debris will settle near the tip).
(2) Release the contents of the dropper onto a white plate in a series of drops. Each successive drop should contain less and less of the unwanted debris. (This is also a good way to isolate small crustaceans, such as a Daphnia or water fleas - more on that later.)
(3) Select a drop with enough fine debris (just visible to the naked eye) to provide targets for setting up the focus and lighting prior to beginning your hunt for "big game" (also attached and feeding protozoa tend to be found on, or around such particles).
Siphon up a suitable drop with an eyedropper and place it on a clean slide (clean the slide each time using moisture from your breath and a tissue). Put a coverslip onto the drop by resting one side against the slide and then slowly lower the other end onto the drop ( a needle is handy for holding up one end of the slip). If there is too much water draw out the excess by touching a piece of tissue to the edge of the coverslip. The less depth the better, as this will prevent organisms from moving out of focus as well as allow the use of high power objectives which have relatively short working distances (i.e., space between the front lens and the object in focus).
It may take 10 minutes or so for some of the more "bashful" inhabitants to show themselves after the shock of slide preparation, so keep looking. As water evaporates from the edge, and bubbles start to form, add more by placing a small drop at the edge of the coverslip (capillary action will quickly draw it under the slip).
Finally, you may want to examine larger animals, such as water fleas or Daphnia. As mentioned in point 2 above you can isolate such animals by the same "drop technique" used to remove unwanted debris. While the animal is looking for a way out of its tiny water "prison" you can prepare the slide. With a tooth pick put three small dabs of Vaseline onto the centre of the slide, leaving enough space at the centre for the drop containing the animal. The Vaseline dabs should be of a height sufficient to hold the coverslip a millimetre or so off the slide's surface.
Now suck up the drop with the animal and place it in the centre of the dabs. Gently lower the coverslip onto the Vaseline, which it should contact just before encountering the drop. Use a tooth pick to slowly press the slip down until the animal is no longer fritting about, but is held stationary by the gentle pressure of the slip. It may take a few tries to learn how much pressure is needed to stop, but not injure the animal. More water can be added at the edge of the slip if needed.
Comments to the author sent via our contacts page quoting page url plus : ('rneumeyer','')">Ron Neumeyer
(Pond image by Maurice Smith)
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