Trapping Living Things in Slides to Look Under a Compound Light Microscope
It might be somewhat hard to focus on small organisms under the compound light microscope because living things sometimes move around too fast for you to view them microscopically. Fortunately, there are some ways to slow down microorganisms so that students could view their movements better under the higher magnification of a compound light microscope. With these methods, it would be easier for the students or children to make some interesting observations on the movements and behaviours of microorganisms while using their optical equipment.
One of the ways to help you focus on living organisms is to use cotton strands. Take tiny strands from a cotton ball and put them in the center of the blank microscope slide. Then put some of the pond water that you have gathered on top of the cotton strands with an eyedropper. Cover all of this with the slide cover slip, but do not push down hard on the cover. Pond and seawater is better viewed microscopically using a well slide. Well slides have mini-wells inside them to make room for the water and allow the microorganisms more space for movement. Since this kind of slide is thicker than the normal ones, though, you have to be careful with looking at them with the high power objective. The objective is long and it may crash into the deep well slide, damaging the slide and the objective. Take note of how you focus the high power objective so that you won’t bring the microscope objective lens into the slide. Actually, most high power objectives such as the 40x and 100x, have spring loaded mechanisms that protect the objective to some degree. Microscopes also have stage travel stops that can be used.
Once you have prepared your slide with the cotton strands, place it on the microscope stage and clip it into place. Start out with a low power objective and look around your sample for the microorganism that you want to focus on. It might be better to dim the lights going through the diaphragm of the microscope so that it would be easier to pinpoint the small organisms in the pond water. Often, if you have too much transmitted light, the specimen is bleached out and hard to see. The iris diaphragm is the part of the microscope that allows you to control the amount of light that enters the objective. Closing down the iris diaphragm produces greater optical depth of field as well as gives greater viewing contrast. Once you have spotted some living microorganisms in the pond water, you could now look at them with a high power objective.
If you’re still having some trouble focusing on the moving microorganism, you could add on a drop of corn syrup on your glass slide. Make sure that you use the white kind of syrup so that it won’t color the sample of pond water on your slide.
Now that you have slowed down the movements of the microorganism that you are going to view under the microscope, it is now time to observe them. You could take down notes on what you have found out. Try to see different kinds of microorganisms inside your sample of pond water and identify them. It might also be interesting to compare microorganisms and how they move.