Lie a cloth on a flat surface such as a table. Place the microscope slide on top of the cloth.
Pour the Horiba zirconia powder out gently onto the slide. Brush off the powder from the slide until only a few zirconia particles remain in the center of the slide.
Plug the microscope into a functioning power outlet. Center the slide on the microscope's table and lock the slide gently with the microscope's latch. Turn the microscope on to view the zirconia particles.
Look into the microscope's eye and adjust the lens until you can clearly see each zirconia particle. Determine the length of one of the particles by measuring it with the lens ruler by eye. Remember that the ruler measures in micrometers.
Convert the micrometers to millimeters, if desired. Note that 1 micrometer equals 0.001 millimeters. Therefore, multiply the measurement in micrometers by 0.001 to give the amount in millimeters. If you measured the Horiba zirconia particle length as 30 micrometers, for example, then the particle's length in millimeters is 0.03.