0 a tool that contains a tube of liquid with an air bubble in it, used to show if a surface is level --
The device serves the same purpose as a spirit level.
The accuracy of a spirit level can be checked by placing it on any flat surface, marking the bubble's position and rotating the level 180.
The instrument requires to be set level (see spirit level) in each quadrant, to ensure it is accurate through a full 360 traverse.
A spirit level on the sight allowed it to be checked.
A spirit level was visible in the view finder, making it easier to avoid camera tilt, one of the pitfalls of stereo photography.
A spirit level was used to monitor for any inclination of the axis to the horizon.
Such precision is routinely obtained with a spirit level and string.
In the middle of the spirit level is a small window where the bubble and the tube is mounted.
