Microscope Light Source & Rheostat
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The amount of light passing through the slide and lenses is controlled in two ways. The condensor (shown here) intensifies light coming from a bulb in the base of the microscope. The condenser can be moved up and down by using a knob (bottom left photograph) adjacent to it, but is generally left at the highest position for optimum performance. The amount of light leaving the bulb and travelling through the condenser is controlled by a dimmer knob (top left photograph).

The light can also be controlled by adjusting the iris diaphragm built in to the condenser. The smaller the opening in the iris diaphragm, the greater the depth of field. This is particularly important when viewing thick slides or depression slides.