In the year 1813, Professor Morichini, of Rome, announced that steel exposed in a particular manner to the concentrated violet rays of the prismatic spectrum becomes magnetic. His experiments, however, having uniformly failed in other hands, had ceased to excite general attention; especially in this country, whose climate is usually so unfavourable for such researches. The unusual clearness of weather last summer, however, induced Mrs. Somerville to make the attempt. Having, at that time, no information of the manner in which Prof. Morichini’s experiments were conducted, it occurred to her, however, as unlikely that if the whole of a needle were equally exposed to the violet rays, the same influence should at the same time produce a south pole at one end, and a north at the other of it. She therefore covered half of a slender sewing needle, an inch long, with paper, and fixed it in such a manner as to expose the uncovered part to the violet rays of a spectrum, thrown by an equiangular prism of flint glass on a panel at five feet distance. As the place of the spectrum shifted, the needle was moved so as to keep the exposed part constantly in the violet ray. The sun being bright, in less than two hours the needle, which before the experiment showed no signs of polarity, had become magnetic; the exposed end attracting the south pole of a suspended magnetic needle, and repelling the north. No iron was near to disturb the experiment, which was repeated the same day, under similar circumstances, with a view to detect any source of fallacy in the first attempt, but with the same result. The season continuing favourable, afforded daily opportunities of repeating and varying the experiment. Needles of various sizes (all carefully ascertained to be free from polarity), and exposed in various positions with regard to the magnetic dip and meridian, almost all became magnetic; some in a longer, some in a shorter time, varying from half an hour to four hours, but depending on circumstances not apparent. The position of the needles seems to have had no influence, but the experiments were generally more successful from 10 to 12 or 1 o’clock than later in the day. The exposed portion of the needle became (with a few exceptions) a north pole, exceptions possibly attributable to some predisposition in the needle, itself to magnetism too slight to be observed. The distance of the needle from the prism was varied without materially varying the effect. It was found unnecessary to darken the room, provided the spectrum was thrown out of the direct solar rays.
Observations of Interstellar Reddening in Cygnus and Perseus
