Why Is a Mechanical Stage Important?
A mechanical stage is a mechanism mounted on the stage that holds and moves the microscope slide. It has two knobs and allows the user to move the slide in the X or Y direction very smoothly and slowly by turning these knobs. This can be very useful at higher magnifications because the slide must be moved in very small amounts and in the opposite direction, or the specimen being viewed will shoot out of the viewing field. Moving a slide by hand at 400x and above can take a lot of practice to get it right!
For example, let's say the user is looking at a single-cell organism that is moving around and heading to the edge of the view field. The organism swims to the right so in order to keep it in view the user must move the slide extremely slowly and, most of us would think you would move the slide to the left. But that's the OPPOSITE direction it should be moved. Due to the optical design of high-power microscopes the slide must be moved to the right (which beginners would think would push the organism even farther away...but it doesn't). Also, if a mistake is made it can be difficult to get the organism back into view since it is so small and the slide must be moved in such small increments. Typically you would then need to switch to a lower magnification objective until you find your organism again, and then start over. This can be very frustrating for the new microscopist!
With a mechanical stage, when the organism moves to the edge, the user simply turns a knob to move the slide very slowly in the desired direction. Even if the knob is started in the wrong direction, it is a simple thing to just reverse the knob and re-center the specimen. Anyone that spends much time using the high power objective should have a microscope equipped with a mechanical stage.