CH. XI.

GALILEO.

85

CHAPTER XI.

SCIENCE OF THE SEVENTEENTH CENTURY.

Astronomical Discoveries of Galileo-The Inquisition force him to deny the Movement of the Earth-Blindness and Death.

Astronomical Discoveries of Galileo, 1609-1642.— The seventeenth century was not many years old when Galileo startled the world with discoveries such as had never been heard of before. He relates that when quite a young man he was so struck with an account given by some of his companions of a lecture on the Copernican theory, that he determined to study it, and he soon became convinced of its truth. Nevertheless, he saw how difficult it would be to prove that the earth moves round the sun, and not the sun round the earth.

When he went to Padua he gave a great deal of time to the study of astronomy, and had already made some remarkable observations, when one day, in the year 1609, being in Venice, he heard that a Dutch spectacle-maker had invented an instrument which made distant things appear close at hand.

This discovery, which Bacon and Porta had foreseen, was made at last almost by accident in Holland, by two spectacle-makers, Zacharias Jansen and Henry Lippershey. It is related that Jansen's children, when playing one day with two powerful magnifying glasses, happened to place

them one behind the other in such a position that the weathercock of a church opposite the house seemed to them nearer and larger than usual, and their father, when he saw this, fixed the glasses on a board and gave them as a curiosity to Prince Maurice of Nassau. Whether this story be true or not, it is certain that in the year 1609, both Jansen and Lippershey made these rough telescopes as toys, though they did not know how useful they might be. But when Galileo heard of it he saw what valuable help it might afford in studying the heavens; and he set to work immediately, and soon succeeded in making a useful instrument.

A diagram of Galileo's telescope is given in Fig. 8. It was made on the same principle as opera-glasses are now, with one convex lens A B, which makes the rays from the object bend inwards or converge, and one concave lens c D, which makes them bend outwards or diverge before they come to a focus. In Fig. 8 one complete cone of rays is drawn coming from the point m, and the outline of another cone from the point n; there are really similar cones coming from all points along the arrow, but it is impossible to give these in a diagram. Each set of rays, as they fall on the lens A B, are made to converge, so that they would end in a point or focus, if they were not caught by the lens C D. But this lens having its surfaces curved inwards makes the rays bend outwards or diverge again, so that the end of the cone m reaches the eye in parallel lines at m' m' and the cone n at n'n'. From the eye, as you will remember (see p. 49), we follow them out in straight lines, and see the image at the angle M o N, so that it appears greatly magnified. If you look at any object through one tube of an opera-glass, and keep the other eye open so as to see the object at its natural distance, you can cover the real image with the magnified one, and thus see the magnifying power

CH. XI.

GALILEO'S TELESCOPE.

87

of your glass. But when you do not compare them in this way you do not realise how much the object is enlarged, because it appears to come nearer, so as to be at some point between M N and o, and consequently to be less magnified. I must warn you that both in this diagram and

A B, Convex lens; C D, concave lens next the eye; m n, real arrow; м N, apparent size of arrow; m' m' and n' n', end of the cones of rays m and n as they reach the eye; Mo N, angle at which the magnified arrow is seen.

the one at p. 95 the proportions are very much distorted, because a star or even a house would be an immense distance off as compared with the length of a telescope, whereas, in the drawing, the arrow must be placed as near to the lenses as they are to each other.

Secondary Light of the Moon.-Galileo's first telescope only magnified three times, that is, made an object three times larger; but he made a second which magnified eight times, and then he turned it to the moon and began to examine the surface of that satellite. He saw the mountains of the moon, and the deep hollows buried in darkness, and the wide plains which he mistook for oceans. Then he noticed that curious light called the secondary light, which

may be seen on the dark side of the moon when only one quarter of it is bright and shining. Galileo discovered that this curious light is a reflection from the earth; for you must know that we reflect the sun's light back to the moon just in the same way as the moon does back to us, and at the time when we see a new moon, the man in the moon (if there were such a person) would see a large full earth, and could wander about at night by earth-light as we do by moonlight. Look up at the new moon just about dusk in the evening, and if it is a clear night you will most likely be able to see a faint outline of the dark side of the moon, which is caused by our earth-light shining upon it.

Jupiter's Moons.-When Galileo had studied the moon and gazed with intense delight on the myriads of tiny stars in the Milky Way, he next turned his telescope to the planet Jupiter. To his great surprise he saw three small shining bodies like stars close to Jupiter, which were quite invisible to the naked eye. Two of them were on the east side of the planet and the other on the west. He waited eagerly for the second night, to see if Jupiter would move away from these stars, but he found them still together, only the two stars which had been on the east side had now moved round to the west, and they were nearer to each other than they had been before. He was quite puzzled as to how this could have happened, and watched and watched for many nights whenever the clouds would allow him; and at last, on the fourth night after he had first seen them, he came to the conclusion that all three stars were moving round and round Jupiter, as the moon goes round our earth. A few nights later he found that there was a fourth star which went round with them; and so Galileo discovered Jupiter's four moons in the year 1610.

This was the first fact in favour of the Copernican theory