You might have come across small glass tubes with three tiny ball bearings  inside, often seen in antique tool collections. These aren’t random curiosities—they are precision parts from bullseye or machinist’s levels, also known as spirit levels. Unlike standard linear levels with a single bubble, bullseye levels use a circular, dome-shaped capsule filled with liquid to measure levelness in two dimensions simultaneously.

In older or specialized European and industrial models, the traditional bubble was replaced with steel ball bearings. These balls roll to the lowest point, offering a clear visual cue of true level from multiple angles. The use of ball bearings, especially in machinery-heavy or high-vibration environments, made them easier to read compared to the bubble-based designs. Some models even used three balls to triangulate levelness more precisely.

You’ll typically find these ball-bearing-filled tubes inside antique wooden, brass, or cast-iron leveling tools, often in machinist toolboxes or at estate sales and flea markets. If the tube is dome-shaped, sealed, and mounted in a metal ring, it’s likely from a leveling instrument. While these tubes are safe to handle if intact, the liquid inside may be flammable or toxic, such as ethanol or oil, so avoid breaking or opening them. If damaged, dispose of them carefully.