An apparatus designed particularly for measuring volume changes due to the state of fullness of the blood vessels of a sharply defined portion of the human finger is described as to construction, mode of operation and working characteristics. The apparatus makes a graphic record of pulse volumes as small as 0.1 cu. mm. and of gradual volume changes as great as 1000 cu. mm. The pulse recorder which employs an optical capsule in which stretching of the rubber membrane is largely avoided shows high sensitivity, low moving mass, responds well to volume changes at a frequency up to 40 cycles per sec. and with diminished amplitude to 60 cycles per sec. and is well damped. The apparatus and method interfere with the body part under study to a minimal extent except through undesirable temp. and humidity of the air in contact with the skin.