Calibrating a microscope requires two things, a stage micrometer and a reticle. Your reticle should already be installed in your eyepiece. In order to calibrate your reticle follow the following procedure.
1. Switch to the lowest magnification of your microscope.
2. Place the stage micrometer onto your microscope with the 0 position on the stage micrometer in line with the 0 position of the reticle (eyepiece micromerter) as seen below:
3. Look for the first spot where the lines of the reticle and the stage micrometer match as they do at the 0 points. In the above image that would be 30. They also mach at 60 as well.
4. Count how many divisions on the stage micrometer there are that match with the 30 spot on the reticle (eyepiece micrometer). In this instance the answer is 20.
5. Since each line is 10um in width 20 of those lines equals 200um.
6. You have all your data to calculate what each division represents. 30 divisions on the reticle (eyepiece micrometer) equals 200 micrometers.
7. Next determine what one division on the reticle (eyepiece micrometer) represents by dividing 200 by 30. This tells us that each division on the reticle under the objective you have selected is 6.67um.
8. Repeat this procedure with all of the other objectives on your microscope.