The controls on the lamp are very simple. There is a power switch, and a brightness control.
The brightness control is important because the aperture diaphragm is used to control the resolution of the image, and should not be used to control the brightness. If the image is too dark or too bright once the condenser has been focused properly and the aperture diaphragm has been set for the best contrast and resolution, then use the brightness control to adjust the amount of light getting to your eyes.
If you are using a camera, you have the option of using the shutter speed to control the brightness as well. If the subject is moving, you will want the brightest light possible, so that you can use fast shutter speeds to prevent motion blur. If you are taking photographs of slow moving live specimens, you might want to reduce the amount of light, to reduce heating the specimen, and use a slower shutter speed.