There are many planes of focus possible, especially for large particles.
One could focus on the bottom, middle or the top of the particle. Smaller
particles surrounding a large particle may not even be observed when focusing
on the top of the particle. Thus it is often important to examine the sample
at various levels. Just as important is the need to be sure one is not
focusing on dust particles on the coverslip, slide, or various optics within
the microscope. It is sometimes worthwhile to first focus on a large brightly
colored sample which is easy to focus on. Below is a mpeg movie focusing
on a large quartz particle. It was taken using the 20x objective.
Click on the link below. If it does not open up the movie, go to the Instruction
Page to find out how to view it.