The Micrograph

A new instrument for observing and recording small changes of mechanical pressure by making use of the phenomena of the interference of light has recently been described. 1 Several important changes in the instrument have been made since the original communication, and the development of its applications has been in steady progress. In the present paper, an account is given of some results obtained on normal subjects in the effort to ascertain the characteristics and possibilities of the instrument. At certain portions of the cardiac cycle, changes take place with such rapidity that it requires an instrument free from any considerable inertia in the moving parts to register them accurately. In instruments heretofore in use, the mass and inertia of the moving parts and the distance through which they are required to move have been considerable. At the points of most rapid pressurechanges in the cardiac cycle, such instruments overshoot the mark on the up stroke and then again on the return because of the inertia. The important feature of the new instrument is that it reproduces