Observations of the sun even with small telescopes and protected by dark glasses are very dangerous to the eyesight. Galileo blinded himself in this way; Sir William Herschel lost one of his eyes; and some modern observers have also suffered. The present writer had a narrow escape from permanent injury while observing the transit of Venus, in 1874, in India, the dark screen before the eyepiece of a 3-inch telescope having blistered--that is, partially fused during the observation. Mr. Cooper, Markree Castle, Ireland, in observing the sun, used a "drum" of alum water and dark spectacles, and found this sufficient protection against the glare in using his large refracting telescope of 13·3-inches aperture. Prof. Mitchell, of Columbia University (U.S.A.), finds that lines due to the recently discovered atmospherical gases argon and neon are present in the spectrum of the sun's chromosphere. The evidence for the existence of krypton and xenon is, however, inconclusive. Prof. Mitchell suggests that these gases may possibly have reached the earth's atmosphere from the sun. This would agree with the theory advanced by Arrhenius that "ionised particles are constantly being repulsed by the pressure of light, and thus journey from one sun to another."[6] Prof. Young in 1870, and Dr. Kreusler in June, 1904, observed the helium line D_{3} as a _dark_ line "in the spectrum of the region about a sun-spot."[7] This famous line, from which helium was originally discovered in the sun, and by which it was long afterwards detected in terrestrial minerals, usually appears as a _bright_ line in the spectrum of the solar chromosphere and "prominences." It has also been seen _dark_ by Mr. Buss in sun-spot regions.[8] The discovery of sun-spots was claimed by Hariotte, in 1610, and by Galileo, Fabricius, and Scheiner, in 1611. The latter wrote 800 pages on them, and thought they were small planets revolving round the sun! This idea was also held by Tardè, who called them _Astra Borbonia_, and by C. Malapert, who termed them _Sydera Austricea_. But they seem to have been noticed by the ancients.