SKILL BUILDER: How to Use a Microscope pt 2: Preparing Slides and Observing Living Specimens
Lesson 16 of 17
Objective: Students will care for live specimens and observe them by preparing their own microscope slides.
This lesson is a follow up to How to Use a Microscope. In this lesson, students learn to prepare their own slides and continue to develop skill with a microscope by observing living specimens which often require manipulation of the x and y axis knobs and the coarse and fine adjustment knobs to both track and focus on the sometimes quickly moving microorganisms students will encounter on their slides.
Connection to Standard:
While so much of the focus on "technology" in the standards seems to mean computer technology, a microscope is inarguably an important piece of technology that broadens the horizons of inquiry for scientists, and as such, is an important tool to gather more information that can be integrated into student writing. The tool itself is complex, so it's proper use requires technical operation and following multi-step instructions.
I begin the lesson by reminding students of the need for safety and care when using the microscopes, both because of the glass slides and the fact that the microscopes are expensive, complicated devices.
I tell students that, in contrast to the previous lesson where we looked at prepared slides of dead specimens, we will be preparing our own slides to observe living specimens. I remind them that they will have to be patient and use their microscopes skillfully to fully appreciate the slides they make and, in a sense, challenge them to be the best microscope operator they can be. Once I’ve given this brief introduction to them, I begin the demonstration of making a slide.
To begin, I hold up a sample of pond water that I’ve collected and walk around from table to table asking students to make observations of the water. Some adjectives that might come out are “gross”, “green”, “nasty”, etc. If a student does not offer the observation, I also point out that there is some sediment settled at the bottom. I ask the class why the sediment is at the bottom and not at the top. Answers will vary, and may include demonstrating the knowledge that the sediment must be more dense than the water, but I’m hoping for someone to mention that the sediment is affected by gravity (an important point later in the demonstration).
I then go to my desk at the front of the room and place a microscope slide on the desk under a document reader and project the image on the screen behind me. I then hold the pond water up with one hand and use a pipette to take a sample of the water. I hold it up for the class and tell them that I am not going to place the water on the slide just yet, explaining that like the sediment in the container holding the pond water, there are organisms and organic matter slowly sinking to the bottom of the pipette. I squeeze the pipette just enough to let a drop hang from the mouth and once a noticeable amount of organic material is in the drop, I place it on the slide.
Once the drop is on the slide, it should be obvious to the students looking at the screen that the drop is not just pure water (this works best if you put a white paper under your slide). I then hold up a cover slip and ask students why we need to cover the drop. Hopefully the students remember that focusing the microscope brings the stage closer to the objective lens and that it could be possible to focus to the point where the water touches, and then adheres to, the lens. Keeping the slide under the document reader, I show students that I place the coverslip at the edge of the drop, perpendicular to the slide itself. I then let the coverslip fall over the water drop to demonstrate that the adhesion of the water will keep the coverslip in place.
An alternative to doing this with a document reader (and perhaps an even better alternative because it's more immediately hands-on), would be to have all students have a pipette with just water, a slide and a coverslip. They could then try each step as you demonstrate at the front of the class.
I then tell students that they will be doing the same thing soon and offer the reminder that, unlike the previous lesson where the subject of their observations just “stood still”, they will likely be observing moving organisms and must remember to constantly adjust the slide using the x and y axis knobs. I also mention that although the water drop seems fairly two-dimensional under the coverslip, to a microorganism, it is quite deep and they will have to use the coarse and fine adjustment knobs to focus on the organisms as they swim back and forth through deeper and more shallow depths of water.
My classroom does not currently have a microscopic camera, but if I did have one, I would also do a short demonstration of the operations necessary to observe the specimens. In this fantasy land of a well-funded school, I would demonstrate:
- how you need to "scan" the slide at low magnification before finding an area that warrants closer inspection
- how increasing the power of magnification on an interesting area of the slide brings even greater rewards
- how moving the slide side to side and back to front (along the x and y axes) allows you to track the lateral motion of the microorganisms
- how adjusting the focus allows you to track the microorganisms as they swim to shallow or deeper sections of the slide)
As it is, without the microscope camera, I help individual groups learn these little tricks of the trade as needed in the guided practice section.
Once the demonstration is complete I ask students to work with a partner and have one partner obtain and set up the microscope. I ask the other partner to approach the lab table next to my desk where I have the pond water sample along with several pipettes, slides*, and coverslips.
*A note on materials: some teachers prefer to use plastic rather than glass slides andcoverslips for economic and safety reasons. While I might use plastic slides for observing cellsin a leaf, a piece of meat, or other static subject, I prefer to use the glass slides in this lesson for two reasons:
1. The plastic slides are very easily scratched and can confuse students unfamiliar with the characteristics of the organisms they might encounter. Nothing worse than telling a student that their beautiful drawing of a giant rod-shaped bacterium is of a scratch.
2. The plastic slides are much thinner than the glass slides and often get stuck under the mechanical stage clips, making it impossible to move back and forth with the x and y adjustment knobs. Again, not so much an issue for a static subject, but it is extremely frustrating for a student to see something interesting and then not be able to track its movement because their slide is too thin.
I then monitor students making their slides and offer my advice if they have any questions or notice them making any particularly egregious errors (such as putting too water that will spill off the slide and onto the microscope, or too much organic material that won't be flattened under the coverslip, etc). For the most part, though, I prefer them to troubleshoot these issues on their own to discover why I mentioned the "single drop with some visible organic material" guidelines in the slide-making demonstration. Again, the practice of preparing slides will ultimately be part of an ongoing investigation of microorganism populations in an aquatic habitat we manage in class, so it’s important that the students be capable of preparing their own slides with minimal assistance from me.
Once all groups have a microscope and a slide set up, I distribute new copies of the Microscope Observation worksheet used in the previous lesson and let students explore their slides. As much as I like them to write observations and draw illustrations of the organisms they encounter, students are often too enthusiastic about the things they’re seeing to remember to also do the written work. At this point, I’m not too worried about it as the point of this lesson is to build skill in operating the microscope and I’m loathe to stand in the way of enthusiasm for science. Still, other students aren’t motivated to use the microscope unless they know the observation sheet counts towards their grade, so to that end, I ask students to complete at least one side of the worksheet. If they want to do the other side, I ask them to either make a new slide or focus on a different area of their slide.
For the remainder of the lesson, I walk from group to group, checking in on their progress and looking through their microscopes momentarily to get a sense of what each group is observing. If I find that some groups happen to have prepared slides with low populations or without much diversity, I sometimes suggest that they prepare a new slide. Students are often excited by what they see and call me or other students over to look at what they discovered with their microscope. Again, the focus of this lesson is on skillbuilding and introducing students to the excitement of observing live specimens so I don’t get in the way of their enthusiasm when it bubbles up.
When we have about 5 minutes left in class, I ask all students to stop whatever they're doing and to send a partner to the front of the class with the slide. I remind them that the slides are glass and to be very careful in handling them and walking with them.
Since these slides have live specimens, I ask students to carefully remove the coverslip and either dip the slides into the pond water (to "send them home") or rinse them off into the sink (to "send them on an adventure"). I then ask them to rinse both the slide and coverslip under running water and set each carefully on a paper towel to dry.
During this time, I ask the other partner to wrap up the extension cord neatly and secure it with a plastic tie. Once the power cord has been neatly secured, I ask them to unfold the dust cover from under the microscope and then place it over the microscope.
Again stressing the need to be careful with the microscopes, I ask students to pick up the microscopes using use one hand on the arm and the other hand under the base of the microscope. I then ask them to return the microscopes from where they took them. Once that is done and I've accounted for all microscopes, class is dismissed.