If an image is too dark under high power what should you use to increase the light intensity

The answer is Condenser. It is a small non non-magnifying lens system that condenses light before it passes through the specimen. Concentrates light on the specimen. Located beneath the stage. First, the iris diaphragm creates an aperture that regulates the amount of light passing through the condenser and ultimately the ocular lenses.  

imagine a line passing through the center of earth that goes through both the north pole and the south pole. this imaginary line is called an axis. earth spins around its axis, just as a top spins around its spindle. this spinning movement is called earth's rotation. at the same time that the earth spins on its axis, it also orbits, or revolves around the sun. this movement is called revolution.