An instrument is described which utilizes the superposition locus of transient pulses traveling in opposite directions on a transmission line for a determination of the interval between the times of generation of the pulses. For testing purposes pulses are generated by discharging a short segment of line through a matching network into a closed loop so that equal pulses start simultaneously in opposite directions around the line. Two systems for pulse detection are described, one requiring repetitive pulsing, in which a detector is coupled to the line at any desired position, yields an output which varies as a function of position, and shows a maximum at the superposition locus; and another, not requiring repetitive pulsing, in which the outputs from multiple fixed detectors are recorded simultaneously. In the latter system the superposition locus appears as a maximum in the output peaks, which are recorded in one-to-one correspondence with the positions of the detectors on the line. A measurement of the refractive index of polystyrene for pulses of duration about 2×10−10 sec. gave 1.48 with the first instrument and 1.45 with the second. The latter value is less certain than the first because of poorer resolution, and both are subject to an upward revision from an undetermined correction in the line propagation rate.