XVII The “Whirlpool” Nebula _M_ 51 in Canes Venatici 210 XVIII The Nebula _M_ 81 in Ursa Major 211 XIX The Nebula _M_ 101 in Ursa Major 212 XX The Nebula _M_ 33 in Triangulum 213 XXI The Lesser and Greater Magellanic Clouds 214 XXII Two Nebulae (N.G.C. 4395, 4401) suggestive of Tidal Action 236 XXIII The twin Nebulae N.G.C. 4567-8 The Nebula N.G.C. 7479 237 XXIV Saturn and its System of Rings 250 INTRODUCTION _The Study of Astronomy_ On the evening of January 7, 1610, a fateful day for the human race, Galileo Galilei, Professor of Mathematics in the University of Padua, sat in front of a telescope he had made with his own hands. More than three centuries previously, Roger Bacon, the inventor of spectacles, had explained how a telescope could be constructed so as “to make the stars appear as near as we please.” He had shewn how a lens could be so shaped that it would collect all the rays of light falling on it from a distant object, bend them until they met in a focus and then pass them on through the pupil of the eye on to the retina. Such an instrument would increase the power of the human eye, just as an ear trumpet increases the power of the human ear by collecting all the waves of sound which fall on a large aperture, bending them, and passing them through the orifice of the ear on to the ear drum. Yet it was not until 1608 that the first telescope had been constructed by Lippershey, a Flemish spectacle-maker. On hearing of this instrument, Galileo had set to work to discover the principles of its construction and had soon made himself a telescope far better than the original. His instrument had created no small sensation in Italy. Such extraordinary stories had been told of its powers that he had been commanded to take it to Venice and exhibit it to the Doge and Senate. The citizens of Venice had then seen the most aged of their Senators climbing the highest bell-towers to spy through the telescope at ships which were too far out at sea to be seen at all without its help. The telescope admitted about a hundred times as much light as the unaided human eye, and, according to Galileo, it shewed an object at fifty miles as clearly as if it were only five miles away. Perhaps it need hardly be said that this is quite insignificant in comparison with the power of modern instruments. The telescope of 100-inch aperture at Mount Wilson, California, the largest at present in existence, admits 2500 times as much light as Galileo’s tiny instrument, and so 250,000 times as much light as the unaided eye. It is hoped that a 200-inch telescope will shortly be built in California; this will admit four times as much light as the 100-inch instrument, or about a million times as much light as the unaided eye.