## Ocular & Stage Micrometers

Calibrating an ocular micrometer is only reliable for one objective on one scope.  Changing the objective lens or using a different scope than your assigned scope may yield varying results.  Always calibrate your micrometers when using new objectives or different microscopes.

Each division of the stage micrometer = .01mm.

Formula:

[(# divisions counted on stage micrometer)(one division of the stage micrometer in mm)/(# divisions counted on ocular micrometer)][1000]

Image Created by David R. Caprette (caprette@rice.edu), Rice University 11 May 2000.  Used with permission.
Retrieved June 2005 from http://www.ruf.rice.edu/~bioslabs/methods/microscopy/measuring.html

Example 1:

.01mm stage micrometer
stage micrometer = 12 divisions to 22 divisions on the ocular micrometer

Step 1:  [(12)(.01)/(22)][1000]

Step 2:  (.12/22)(1000)

Step 3:  (.0055)(1000)

Equals:  5.5µm per ocular division

Example 2:

.01mm stage micrometer
stage micrometer = 1 divisions to 7 divisions on the ocular micrometer

Step 1:  [(1)(.01)/(7)][1000]

Step 2:  (.01/7)(1000)

Step 3:  (.0014)(1000)

Equals:  1.4µm per ocular division

Problem 1:

You have calibrated your ocular micrometer.  Each division of the ocular micrometer equals 1.4µm.  You count 5 divisions of the ocular micrometer as the length of an individual specimen.  How long is this specimen?

Formula:  Total specimen length (x) = (length of one division of the ocular micrometer)(number divisions counted as length of an individual specimen).

Step 1:  x = (1.4µm)(5 divisions)

Step 2:  x = 7µm

Note:  In unknowns, if your organism is 7µm in length, does that length fall within the known lengths for the species you think you have?

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