Munro (1950) deduced from ground-based observations that traveling ionospheric disturbances (T.LD.'s) are large-scale distortions of the electron density contours, which travel through the ionosphere, and Calvert and Schmid (1964) attributed extra traces and distortions of the main echo traces, which occur near the critical frequency of topside ionograms, to refractive effects associated with large-scale irregularities. In an examination of topside and sub-peak ionograms recorded close in position and time, it was found that on a number of occasions refractive effects occurred on topside ionograms, when manifestations of T.LD.'s were observed on sub-peak ionograms (Dyson 1967b).