The first announcement of the construction of this important instrument appeared almost paradoxical: a piece of calcareous spar was to be cut in two, the surfaces of the section polished, and then reunited by help of Canada Balsam: and it seemed strange that from such an operation there should have resulted any change in the optical properties of the mass. Even now that the instrument has been in use for some time, the true nature of its performance is often misunderstood ; while no investigation has been made public, the object of which is to enquire into the laws of the action, and into the circumstances which determine the peculiar forms of the parts.