in structure and hence, in solution or suspension, give rise to significant electric birefringence signals. Furthermore, there have been three electronic innovations which have greatly eased the experimental measurement of the effect for such materials. These were the development of photomultiplier tubes for detection, of oscillo­ scope~ for display and of high voltage generators developing bursts or pulses of potential difference. The last mentioned enable the experi­ menter to study the Kerr effect not only for its amplitude but also in the time domain. The rates of molecular response to the switching of the electric field lead directly to information on the size and geo­ metry of the constituent molecules and particles in a dilute solution or suspension.