An apparatus is described which facilitates the measurement of ultrasonic fields within transparent solids. This optical system makes use of the birefringence produced within the solid by the passage of an ultrasonic wave. The interaction of light with ultrasonic fields is discussed and expressions derived for the modulation of light by both transverse and longitudinal ultrasonic waves. Various detection systems are discussed and the reasons for the choice of a system, making use of a wide slit in conjunction with a pulsed ultrasonic source, are given. An electronic apparatus, assembled for the most part from commercially available components to implement the measurement, is described. The output of the system is a strip chart recording of the strength of the ultrasonic field along a line. The advantages and limitations of the electronic system are discussed. A typical measurement made with the apparatus is shown and suggestions are given to extend the range of the apparatus.