THE method of studying the ionized regions of the upper atmosphere by emitting small pulses of electric waves and recording the echoes reflected from the ionosphere is now well-known and is in everyday use in many parts of the world: The commonly recognized reflecting regions are at heights of 80 km. and above. Communications to the correspondence columns of NATURE of May 9 and 23, 1936, described observations indicating the existence of reflecting regions at much lower heights. These new regions were first discovered in May 1935, and an account of their investigation over a period of about a year from that date was given in a paper presented on February 4 to the Royal Society by R. A. Watson Watt, A. F. Wilkins and E. G. Bowen. A summary of the paper will be found on page 299. It appears from this work that there may be three electrified regions in the middle atmosphere at average heights of 10, 40 and 60 km., which are capable of reflecting radio waves of frequencies of the order of 6 megacycles per second and above (wave-lengths 50 metres and below). The lowest region has been found to be stratified to such an extent as to indicate the existence of five distinct layers at heights between 8.5 and 13.5 km., with reflection coefficients as high as 0.7. The observations so far made do not indicate any marked diurnal or seasonal changes. An interesting feature of the investigation so far conducted is that echoes from these layers have been recorded at sensibly vertical incidence at wavelengths at and below those now in use for the television service. The existence of such echoes would cause the picture seen in a television receiver to have a doubled or blurred appearance. There is, however, insufficient evidence available to state how serious this possibility may be in the development of television services on the wave-lengths at present in use.