If you order for the whole lab you will get a better price than by ordering by ones and twos. Some were on the base of the frame will be the on-off switch and light intensity control. These may be one dial or slider. The electronics of a microscope controls the light intensity. The condenser diaphragm controls the contrast not the intensity.
Most microscopes change intensity by changing the voltage to the lamp.
This raises or lowers the intensity of the lamp. It also changes the color of the light that the lamp produces. Reducing the voltage makes the light more red while increasing it makes the light more blue. Mostly this isn't a problem, a blue filter will usually be adequate for most viewing. If your taking color pictures it can be a real problem.