Microscope Activities, 11: Using a Mechanical Stage
In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.
EXPERIMENT 11: Using the Mechanical Stage Scales to Measure and Relocate an Object of Interest, and to Perform Statistical Analyses
To use the mechanical stage scales for measuring the size of microscopic specimens, to relocate objects of interest, and to perform statistical analyses.
Locate the mechanical stage coaxial knurled control knobs beneath and to the right rear of the stage (Figure 11-1).
Rotate these coaxial knobs, and note that they move the specimen left and right for a graduated scale distance of 140 mm, and forward and back for a graduated scale distance of 40 mm; both mechanical stage scales have an associated vernier scale (Figure 11-2).
The vernier scale consists of ten divisions whose total length equals nine divisions on the main scale; this allows the reading of main scale divisions (millimeters) to within one-tenth of a division (0.1 millimeter).
To read the scale in Figure 11-3, you go to the zero reading on the vernier, which is marked by a white dot at the end of the line, and note that the fiducial line with the white dot falls between 25 and 26 on the main scale—but closer to 26. Write down the lower number—25—on a piece of paper. Next, examine all of the vernier divisions and find the one that exactly lines up with a main scale division—in this case, it is the seventh vernier division; that is the number of tenths of a millimeter; add this number to 25, for a final reading of 25.7 mm.
Since you can now read both mechanical stage scales to within 0.1 mm (100 μm), you can use the scales to measure the dimensions of objects. To do this, line up the tip of your eyepiece pointer on one side of a microscopic object, and take a stage scale reading. Move the mechanical stage control knob until the other side of the object is lined up with the tip of the eyepiece pointer, and take another reading; the difference between the two scale readings is the dimension of the object, in millimeters. Only one scale is used to make this measurement, so that if your object does not traverse N/S or E/W, remove the slide holder by unscrewing and removing the two knurled thumbscrews, and then freehand orienting the microscope slide so that the object of interest does move N/S or E/W when the mechanical stage control knob is rotated.
Besides measuring the dimensions of microscopic objects, the two stage scales can be used to relocate an object of interest. Suppose you are looking at a stained blood smear, and come across a leucocyte (white blood cell) with a Barr body, and wish to record the location of this genetic sex determinant for later discussion. Rotate your eyepiece so that the pointer is vertical, and place the white blood cell with the Barr body right on the tip of the pointer. Then read both scales, using the verniers, and record these coordinates; if you wish, you may record these two readings in the form of a fraction; say, V/H (vertical/horizontal) e.g., 21.4/102.5. Later, when you want to find this same cell again, put the microscope slide in the slide holder, set the two scales to the previously recorded coordinates, and look in the microscope; if not precisely at the tip of the eyepiece pointer, the cell will be very close to it.
Still another use for the mechanical stage scales is for performing statistical analyses. For example, you may want to determine the percentage of quartz grains in a given rock thin section, or the number of fibers on a membrane filter that represents a liter of air, etc. In this case, depending on the objective in use, you start by setting some even number on both stage scales, and making your count in the field of view. Then move the horizontal scale by some specific incremental amount; make another count; again set the scale for the same incremental amount and count…until you get to the end of the sample, or your horizontal scale. Then you move the vertical scale by some incremental amount and again traverse the slide horizontally in the other direction until you get to the end—and so on, until a statistically valid number has been obtained.
Set your two mechanical stage scales at some randomly selected position. Read and record both stage scale settings to 0.1 mm. Next, you and your lab partner exchange places, and read and record the other’s stage scale settings. Compare the results.
Focus on any sample, and select some particular structure of interest. Place this object at the tip of your vertically-oriented eyepiece pointer. Read and record the vertical and horizontal coordinates. Upset the two mechanical stage settings, and supply your lab partner with your previously recorded coordinates; have your lab partner make these settings, and check if the object of interest that you chose is centered in the field of view.
It might be interesting to see if these coordinate settings are transferable between different microscopes from the same manufacturer; it will be a test of consistency in manufacture of the instruments.
Since such coordinate readings are impossible between microscopes of different manufacturers, and between different models by the same manufacturer, professional microscopists utilize finder slides, which are gridded, alphanumeric slides referenced to one corner. Two microscopists in different locations must both possess finder slides; one mails a slide with the coordinates to the other; the second user places their finder slide on the stage, locates the supplied coordinates, and substitutes the slide with the object of interest, which will then be in the field of view.
Note: Many, if not most, student-grade microscopes are not supplied with a built-in mechanical stage; there may be only two stage clips (spring clips) present to hold the microscope slide in place. Also, when a mechanical stage is built in, it does not necessarily have graduations on it; it may be plain, and intended only to move a specimen slide about, and not for quantitative use.
Even for plain square stages, however, attachable mechanical stages are available, as illustrated in Figure 11-4.
The attachable mechanical stage is slipped over the edge of the square stage, and secured with the two thumbscrews shown in the illustration. The attachable mechanical stage shown is graduated, but others may be plain.
Finally, it should be mentioned that experienced professional microscopists using rotating, graduated stages on polarizing microscopes tend to use neither stage clips nor a mechanical stage, preferring to make all movements of the slide freehand—except when particle size analyses or other quantitative, systematic surveys of a sample have to be made, in which case, an attachable graduated mechanical stage is then installed.