SECTION - 15

Measuring With the Microscope

Eyepieces fitted with Micrometer Disc (Micrometer Eyepieces) are used for measuring the linear dimensions of microscopic objects. Micrometer Discs are scales on glass plates which fit into standard Eye-pieces. They can be purchased separately or already installed in the Balplan Eyepiece. To install a Reticle in a non-focusable Eyepiece, unscrew the Diaphragm, place the Reticle on the Diaphramg with pattern side down and crimp the edge of the mount in about four places to hold the Reticle in place. Screw the Diaphragm into the Eyepiece until the Reticle is in sharp focus. To install a Reticle in the Focusable Eye-piece, Cat. No. 31-15-67, unscrew the focusable eye lens, drop the Reticle onto Diaphragm and install spring ring on top of the Reticle to make secure. Replace Eye-piece lens and focus on Reticle.

Before measurements can be made with any Eyepiece Micrometer Disc, it must be calibrated for the particular Objective, Eyepiece, and tube length employed.

This consists of determining the magnification factor, or the value of a division on the Eyepiece scale for a known dimension in the specimen plane. For this purpose, a Stage Micrometer is required. This is usually a glass slide carrying a scale of known intervals.

Two methods for calibration are presented below.

MAGNIFICATION FACTOR METHOD

When both Stage Micrometer and Eyepiece Micrometer are graduated in the same system, it is very easy to determine the number of times an object is magnified by the Objective and Field Lens of the Eyepiece when focused in the plane of the Eyepiece Micrometer Disc. Therefore, the size of any object as shown on the Eyepiece scale will be that dimension divided by the magnification factor. To determine the magnification factor, divide the dimension subtended in the Eyepiece scale by the actual dimension on the Stage Micrometer scale included. Thus, if 0.1mm on the Stage covers 1.86mm on the Eyepiece scale, the magnification factor is 1.86 - 0.1 = 18.6. If an object subtends 0.25mm in the Eyepiece, its actual size is 0.25mm 18.6 = 0.0134mm.

EYEPIECE SCALE VALUE METHOD

If the value of the Eyepiece Micrometer scale is not known, simply determine the number of divisions in the Eyepiece scale subtended by a known value on the Stage Micrometer when in focus.

To determine the value of one Eyepiece scale division, simply divide the value of the Stage Micrometer division by the number of Eyepiece scale divisions which it subtends in the image.

For example, 0.1mm on the Stage Micrometer scale covers 18.6 divisions on the Eyepiece scale. Therefore, one division of the Eyepiece scale equals 0.1mm 18.6 or 0.00537mm. The size of an object sub-tending 2.5 divisions in the Eyepiece will be 3.5 x 0.00537 = 0.0134mm.

In calibrating the instrument and in making measurements, it is important that the object and the Eyepiece Micrometer scale appear to lie exactly in the same plane. Move your head slightly from side to side and observe the apparent motion of the image relative to the Eyepiece scale. When the image is focused in the plane of the Eyepiece scale, no relative motion will be seen.

Because your Microscope may have a small amount of residual distortion, it is a good practice, when making precision measurements, to calibrate the Micrometer Scale for only that portion of the scale being used.

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