With regard to their size, spots vary as much as they do in their duration. The majority of them are telescopic, that is, are only visible with the aid of a telescope; but instances are not uncommon of spots sufficiently large to be visible to the naked eye. The ancients knew nothing about the physical constitution of the Sun, and their few allusions to the subject were mere guesses of the wildest character. They were, however, able to notice now and then that when the Sun was near the horizon certain black spots could sometimes be distinguished with the naked eye, but they took these for planets in conjunction with the Sun, or phenomena of unknown origin. Earliest in point of date of those who have left on record accounts of naked eye sun-spots are undoubtedly the Chinese. In a species of Cyclopædia ascribed to a certain Ma-touan-lin (whose records of comets have been of the greatest possible use to astronomers), we find an account of 45 sun-spots seen during a period of 904 years, from 301 A. D. to 1205 A. D. In order to convey an idea of the relative size of the spots, the observers compared them to eggs, dates, plums, etc., as the case might be. The observations often extended over several days; some indeed to as many as ten consecutive days, and there seem no grounds for doubting the authenticity of the observations thus handed down to us. A few stray observations of sun-spots were recorded in Europe before the invention of the telescope. Adelmus, a Benedictine monk, makes mention of a black spot on the Sun on March 17, 807. It is also stated that such a spot was seen by Averröes in 1161. Kepler himself seems to have unconsciously once seen a spot on the Sun with the naked eye, though he supposed he was looking at a transit of the planet Mercury. None of these early observers have told us the way in which they made their observations, but the smallest of boys who has any claim to scientific knowledge is aware of the fact, that by the use of so simple an expedient as a piece of glass blackened with smoke, spots which are of sufficient size can be seen with the naked eye. Before telescopes came into use it was customary to receive the solar rays in a dark chamber through a little circular hole cut in a shutter. It was thus that J. Fabricius succeeded in December 1610 in seeing a considerable spot and following its movement sufficiently well to enable him to determine roughly the period of the Sun’s rotation.