Rack-and-pinion

In many stereoscopes, a rack-and-pinion mechanism is used to adjust the focus of the lenses. This mechanism allows the distance between the lenses and the stereoviews to be adjusted so that the images appear sharp to the observer.

A rack-and-pinion system consists of two main components. The rack is a straight bar with a series of evenly spaced teeth. The pinion is a small gear that meshes with these teeth. When the pinion is rotated—usually by turning a focusing knob—the gear moves along the rack. This converts the rotational motion of the knob into linear movement.

In a stereoscope, the rack is typically attached to the sliding lens holder, while the pinion is connected to the focusing knob. Turning the knob causes the lens assembly to move forward or backward along a guide. This movement adjusts the distance between the lenses and the stereoview.