2. Place the microscope on a flat surface that is clear of objects. The arm should be toward you.

3. Look through the eyepiece. Adjust the diaphragm so that light comes through the opening in the stage.

4. Place a slide on the stage so that the specimen is in the field of view. Hold it firmly in place by using the stage clips.

5. Always focus first with the coarse adjustment and the low-power objective lens. Once the object is in focus on low power, turn the nosepiece until the high-power objective is in place. Use ONLY the fine adjustment to focus with this lens.

6. Store the microscope covered.

SI Measurement
The International System (SI) of Measurement is accepted as the standard for measurement throughout most of the world. Four of the base units in SI are the meter, liter, kilogram, and second. The size of a unit can be determined from the prefix used with the base unit name. For example, kilo means one thousand; milli means one-thousandth; micro means one-millionth; and centi means one-hundredth. The tables below give the standard symbols for these SI units and some of their equivalents.

Larger and smaller units of measurement in SI are obtained by multiplying or dividing the base unit by some multiple of ten. Multiply to change from larger units to smaller units. Divide to change from larger units to smaller units. For example, to chamge 1 km to meters, you would multiply 1 km by 1000 to obtain 1000 m. To change 10 g to kilograms, you would divide 10g by 1000 to obtain 0,01 kg.

The top of the thermometer is marked off in degrees Fahrenheit (ºF). To read the corresponding temperature in degrees Celsius (ºC), look at the bottom side of the thermometer. For example, 50ºF is the same temperature as 10ºC. You may also use the formulas shown here for conversions.

Conversion of Fahrenheit to Celsius:  ºC = 5/9(ºF - 32)

Conversion of Celsius to Fahrenheit:  ºF = (9/5ºC) + 32)

Forming a Hypothesis
Suppose you wanted to earn a perfect score on a spelling test. You think of several ways to accomplish a perfect score. You base these possibilities on past experiences and observations of your friends' results. All of the following are hypotheses you might consider that could explain how it would be possible to score 100 percent on your test:

If the test is easy, then I will get a good grade. If I am intelligent, then I will get a good grade. If I study hard, then I will get a good grade.

Scientists use hypotheses that they can test to explain the observations they have made. Perhaps a scientist has observed that fish activity increases in the summer and decreases in the winter. A scientist may form a hypothesis that says: If fishes are exposed to warmer water, their activity will increase.

Designing an Experiment to Test a Hypothesis
Once you have stated a hypothesis, you probably want to find out whether or not it explains an event or an observation. This requires a test. To be valid, a hypothesis must be testable by experimentation. Let's figure out how you would conduct an experiment to test the hypothesis about the effects of water temperature on fishes.

First, obtain several identical, clear glass containers, and fill them with the same amount of tap water. Leave the containers for a day to allow the water to come to room temperature. On the day of your experiment, you fill another container with an amount of aquarium water equal to that in the test containers. After measuring and recording the aquarium water temperature, you heat and cool the other containers, adjusting the water temperatures in the test containers so that two have higher temperatures and two have lower temperatures than the aquarium water temperature.

You place a guppy in each container. You count the number of horizontal and vertical movements each guppy makes during five minutes and record your data in a table. Your data table might look like this:

From the data you recorded, you will draw a conclusion and make a statement about your results. If your conclusion supports your hypothesis, then you can say that your hypothesis is reliable. If it did not support your hypothesis, then you would have to make new observations and state a new hypothesis, one that you could also test. Do the data above support the hypothesis that warmer water increases fish activity?

Separating and Controlling Variables
When scientists perform experiments, they must be careful to manipulate or change only one condition and keep all other conditions in the experiment the same. The condition that is manipulated is called the independent variable. The conditions that are kept the same during an experiment are called constants. The dependent variable is any change that results from manipulating the independent variable.

Scientists can only know that the independent variable caused the change in the dependent variable if they keep all other factors the same in an experiment. Scientists also use controls to be certain that the observed changes were a result of manipulation of the independent variable. A control is a sample that is treated exactly like the experimental group except that the independent variable is not applied to the control. After the experiment, the change in the dependent variable of the control sample is compared with any change in the experimental group. This allows scientists to see the effect of the independent variable.

What are the independent and dependent variables in the guppy experiment? Because you are changing the temperature of the water, the independent variable is the water temperature. Because the dependent variable is any change that results from the independent variable, the dependent variable is the number of movements the guppy makes during five minutes.

What factors are constants in the experiment? The constants are using the same size and shape containers, filling them with equal amounts of water, and counting the number of movements during the same amount of time. What was the purpose of counting the number of movements of a guppy in an identical container filled with aquarium water? The container of aquarium water is the control. The number of movements of the guppy in the aquarium water will be used to compare the movements of the guppies in water of different temperatures.

Why is it important to know the best temperature for a fish to survive? If you have an aquarium at home, you have probably learned how different fishes need different conditions to survive. They probably came from many different parts of the world and are adapted to living in very different environments.