This device operates on opto-mechanical principles. A tracking ball contacts two rollers, set at 90 degrees to one another. These rollers are fitted with slitted wheels which turn as the rollers turn. A light source for each wheel shines through the slits and is detected by photoreceptors on the other side. As the wheel turns, light shinning through the slits is registered by the photocells as blips. These sequences are digitized and translated by the computer as horizontal and vertical movements of the cursor on the screen.
