100 Years Ago

Dr. Carl Lumholtz, the Norwegian explorer, who for the past five years has been travelling in the hitherto unknown regions of North-Western Mexico for the American Museum of Natural History, lectured before the Geographical Society in Christiania on September 12 and gave a description of his life and travels among the wild Indian tribes of the Western Sierra Madre... In order to study these interesting people he sent back the entire staff of his expedition and lived alone among them. At first the tribes objected to his taking up his abode in this way, but eventually he gained their confidence and was allowed to remain. He learnt their ways, their language and their songs, and joined in their dances... They are very intellectual, and are, according to Dr. Lumholtz, a far superior race to their kinsmen in the United States and South America. Among many of the tribes he found a higher degree of morality than in civilised countries.

From Nature 19 September 1901.

50 Years Ago

It is impossible to predict precisely the course of a great aurora, but what usually happens is as follows. At first, a glow, much like dawn (hence the name aurora borealis, northern dawn, which was first used by the French philosopher, Gassendi, in 1621), appears on the northern horizon... The glow then ascends from the horizon to form an arc, the lower border of which is at a height of about 100–110 km... The true colour of most arcs is probably green, for a predominant line in the spectrum is the green forbidden line 5577 A. of atomic oxygen, and this happens to be in the region of maximum sensitivity of the retina. But, except in the case of very bright arcs, it appears to the eye to be grey-white, since the light intensity is below the threshold of colour perception. The arc may then brighten suddenly to an intensity above the threshold such that patches of vivid greens and reds can be seen here and there along its length. Simultaneously the lower border becomes extraordinarily sharply defined and is appreciably lower, all of which are signs that the arc is about to lose its regular shape and to show ray structure. It may then undulate to appear like the folds of a huge waving curtain, or it may take the form of a wide band, stretching irregularly across the sky.

From Nature 22 September 1951.