The compensating plate was at the heart of the first interferometers developed at the beginning of the nineteenth century. In essence, it is a humble glass plate that could be pivoted around an axis, changing the angle of incidence of the incoming beam of light onto it. The apparent simplicity of the contraption hides its ingenuity: by turning the compensating plate around, the experimenter could increase or decrease the length of the path taken by light while moving within the glass, thus delaying it more or less on each arm of the interferometer, until conditions for interference could occur again. Specifically, the technique developed by the first interferometrists consisted of bringing the central maximum of interference back to its original position and then measuring the angle along which the compensating plate was turned, thus allowing for the calculation of whatever quantity was varied within the system. This presentation aims at showing both the origins of the idea of the compensating plate and the uses made of it in the beginning of interferometry, displaying how the success achieved by these pioneering instruments was instrumental not only for the development of the next generations of interferometers, but also for the establishment of the undulatory theory of light itself. These instruments also epitomize a transition period in the history of instrument making in which artisans and academicians worked closely together in the development of their measuring instruments.