Put the slide on the stage of the microscope and put it into focus under low power. Low power is the easiest magnification by which to place the object in focus. If the object appears blurry, use the coarse-adjustment knob to bring the stage of the microscope closer to the low-power objective lens. You can then make further adjustments by rotating the fine-adjustment knob. The fine-adjustment knob is smaller than the coarse-adjustment knob. It is necessary to have the object in focus before accurately estimating the size under the microscope.
Use a ruler to measure the diameter of the field of view. Look through the eyepiece while measuring the diameter. Place one end of the ruler at the end of the field of view. Millimeters are a convenient unit of measurement. The diameter of the field of view is also often measured in micrometers. To make the calculations easier, you can convert the units to micrometers after.
Estimate how many times the object appears over the field of view of the microscope. To do this, place a piece of graph paper under the object. Look through the eyepiece while performing this estimation, otherwise you will not obtain accurate results. Use the graph paper to determine how many squares of graph paper the object occupies. Then count the total number of squares across the field of view. For example, if the field of view contains a total of six graph paper squares, and the object covers a length of about two graph paper squares, then the object fits across the field of view at least three times.
Divide the diameter of the field of view by the total number of times the object under the microscope fits across the field of view. For example, if you measured the diameter of the field of view to be 18 millimeters, and the object appeared to fit across the field of view three times, then you would divide 18 millimeters by three. The resulting answer would be six millimeters. Therefore, the size of the object under the microscope would be approximately six millimeters. To convert your answer to micrometers, multiply it by 1,000 because there are 1,000 micrometers in one millimeter. Therefore, if you received an answer of six millimeters, your answer in micrometers would be 6,000.