THE largest group of sunspots since January 1940 crossed the sun's disk between September 10 and 23. The group was a complex stream of spots some 150,000 miles in length, and its maximum area exceeded 2,000 millionths of the sun's hemisphere, or about 2,300 millions of square miles. At central meridian passage on September 16.8, the centre of the group passed within 4° of the centre of the sun's disk. Thus the earth was in a favourable position to be affected by a corpuscular stream that might be shot out from this disturbed region within a day or two of September 16–8.. Statistical data of sun–spots and magnetic storms show that out of every ten spot–groups of great size (1,500 millionths of the sun's hemisphere or greater), seven groups are associated near the time of their central meridan passage with a magnetic storm, the mean position of the group at the time of the commencement of the storm being about one day past the central meridian (see the Observatory, 62, 319; 1939). A valuable criterion of especially active spot groups is the occurrence of brilliant chromospheric eruptions, or ‘solar flares’ (loc. tit., p. 321), which may be observed in monochromatic light at certain wavelengths only—the solar spectrum lines generally used being C for visual observations and H and K for photographic records.