To verify that any ruler or measuring scale is accurate, one needs to compare it to the standard. The simplest way to make this comparison might be to lay the scales together and note any discrepancies, but this method is not very sensitive. Measurement experts are continually searching for ways to improve the sensitivities of these comparisons between the working scale and the standard. Instruments that are used to make these comparisons --often called "comparators"-- have evolved with higher and higher sensitivity, that is better resolution.
Hassler used this microscopic beam compass for transferring units of length from one scale to another and for comparing them. The instrument's retractable lenses magnified its points, thus making it easier for him to position them. Hassler's microscopic beam compass has a stylistic similarity to another of his compasses. Hassler listed among the purchases that he made for the Government, "Two beam compasses with short and long rods, and a double set of points, and one set to work upon brass, by Fidler." Neither of the two compasses are inscribed with a maker's name, but it is possible that they are the Fidler compasses. In 1825, Hassler stated that the beam compass "will soon be found inadmissable for standards work."
Follow a path that traces the historical development of length comparators.