Measuring Techniques D3 Part III: Measuring the Size of a Specimen Using a Stage Micrometer and an Ocular Micrometer

Preface: Before beginning this module, you should be familiar with the parts, use, and care of the compound microscope. If you are unsure about the microscope’s operation, please review the Use of the Microscope module. Mishandling of the microscope can lead to damage of the instrument.

Module Requirements: To complete this module, the following equipment will be provided to you by an SLC assistant. -microscope with ocular micrometer -prepared microscope slide -stage micrometer -worksheet If materials are missing or you are having problems getting started, please ask one of the SLC staff for help.

Notes The Stage and Ocular Micrometer Technique is the most precise method used for measuring microscopic specimens here at the Science Learning Center. An ocular micrometer consists of an ocular lens with equal space gradations that, unlike the stage micrometer, have no units. Look into the microscope at your workstation to make sure you see the ocular micrometer. If you don’t see the micrometer, contact a Student Assistant.

Notes If you have completed module D1, you should be familiar with the stage micrometer. The modules here at the Science Learning Center use a 2mm micrometer. In lab situations though, always check the label for the calibration.

Position the stage micrometer on the microscope’s stage and clearly focus the scale using the 10X objective lens. Note: If you are having trouble finding the scale at 10X, switch to a lower power, such as 4X, to center scale. Then select the 10X objective and use the fine adjustment to focus.

Notes Though stage micrometers are very accurate, we cannot place both a specimen on a slide and the micrometer under the microscope at the same time. To solve this dilemma, we can make use of the ocular scale. It’s important to remember that the ocular micrometer divisions have no units, and that each division will change with each magnification. Therefore, you must calibrate the ocular scale before you can use it.

Notes To calibrate the ocular scale, use the stage micrometer. Your calibration will determine how many millimeters are represented by one division on the ocular scale, or one ocular division. If you find that one ocular division equals 0.3mm, then you know two divisions equal 0.6mm and so forth. Therefore, if you find a specimen covers three ocular divisions, you will know that it measures 0.9mm in length.

Calibration With the stage micrometer in focus, rotate the eyepiece until the ocular micrometer aligns with the stage micrometer. This example shows that one ocular division (od) equals 0.1mm.

Example Using the ocular division from our last slide, we can measure the size of this animal. Based on what you have learned, can you figure out its length?

Example The simple equation can be written as: This animal occupies 3.4 ocular divisions. We found that 1 od is equal to 0.1 mm. So the size of this animal is 0.34mm. The simple equation can be written as: (# of ocular divisions) x (# of mm per division) = diameter of cell in mm

Notes The last slide gave a quick example. Do not take for granted that all ocular micrometers are calibrated the same. Always calibrate your instrument before making your measurements.

Example Suppose your microscope had an ocular lens where one division equaled 0.02mm. You found that your specimen covers 3 ocular divisions. What is the length of your specimen? (3) x (0.02mm) = 0.06mm

Try it! Follow the procedure with the practice slides provided. Remember it’s good practice to calibrate your microscope for each specimen.

Module Completed! Please return to the main desk to obtain a post test from your friendly Science Learning Center personnel.