New types of levelling instruments using reversible bubbles

The paper describes the properties of reversible bubbles which can be used both graduated and ungraduated. Making use of certain of such properties, it can be shown how a true level can be obtained as the mean of two observations from a single station in an extremely simple manner. The theory is then applied to the design of various types of self-adjusting levels. A series of actual levels, following the author's theory and specially made for the paper, are illustrated. A specification for an ideal level, using the same theory, is given. The application of a reversible bubble to the adjustment of a collimator is also dealt with.