Galileo rose from his knees it is said that he stamped his foot and whispered to a friend: E pur si muove' ('Nevertheless it does move').

After a time he was allowed to go back to his own home, but never again to leave it without the Pope's permission. He went on with his studies, and made many useful observations; but in the year 1636 his sight began to fail, and he soon became totally blind. At this time he wrote to an acquaintance these touching words: 'Alas! your dear friend and servant has become totally and irreparably blind. These heavens, this earth, this universe, which by wonderful observation I had enlarged a thousand times beyond the belief of past ages, are henceforth shrunk into the narrow space I myself occupy. So it pleases God, it shall therefore please me also.' He died January 28, 1642, in his seventy-eighth year; having accomplished his work. In spite of all opposition, his discoveries had firmly established the truth of the Copernican system of the universe.

Chief Works consulted.-Brewster's 'Martyrs of Science;' Drinkwater's 'Life of Galileo;' Herschel's 'Astronomy;' Whewell's 'Inductive Sciences;' 'Enclyclopædia Britannica,' art. 'Astronomy;' Baden Powell's 'Hist. of Natural Philosophy;' Ganot's 'Physics,' edited by Atkinson.

Kepler the German Astronomer-Succeeds Tycho as Mathematician to the Emperor Rudolph--His description of the Eye-He tries to explain the orbit of the planet Mars-—And by comparing Tycho's tables with observation discovers his First and Second Law of the movements of the Planets-His delight at Galileo's discoveriesKepler's Third Law-Comparison of the labours of Tycho, Galileo, and Kepler.

Kepler, 1571-1630.-While Galileo was occupied in discovering unknown worlds with his telescope, another famous astronomer, named Johannes Kepler, was working out three grand laws about the movements of the planets. John Kepler was born in 1571. His parents, though noble, were poor, and always in difficulties, but in spite of all obstacles he managed to educate himself, and even to take his degree at the University of Tübingen. In 1594 he was made Professor of Astronomy at Gratz, in Styria, and while there he began his attempts to discover the number, size, and orbits of the planets, but at first with no success. In 1597, when the Catholics at Gratz rose against the Protestants, Kepler, being a Protestant, was forced to leave the city, and would have been in great difficulties if his friend Tycho Brahe had not invited him to come to Prague as his assistant in the observatory. Here Kepler worked with Tycho at his astronomical tables, called the 'Rudolphine Tables,' in honour of the Emperor Rudolph; and when Tycho died,

in 1601, he succeeded him as principal mathematician to the Emperor.

Kepler on Optics, 1604.-Although Kepler is chiefly known as an astronomer, his first work, published in 1604, was on Optics, and in it he points out most beautifully the true use of the different parts of the eye. He was much struck with Porta's idea that the eye is like a camera obscura, and he proved that the rays of light, after passing through the lens of the eye, form a real picture upside down on the fine network of nerves called the retina, at the back of the eye, and are then conveyed by the optic nerve to the brain. He also pointed out that the reason why we do not see things upside down is that since our mind follows out each ray in a straight line, the rays appear to cross back again on the lens of the eye, and we see them as if they had never been inverted. This is, however, a question still undecided by physiologists.

Kepler invented a much more powerful telescope than the one which Galileo had made. You will see by turning back to p. 88 that the fault of Galileo's telescope was that it made the rays diverge or bend outwards, just as they reached the eye, and in this way many of them passed outside and were lost. Kepler avoided this by using two convex lenses. In his telescope (see Fig. 9), the rays from the object m n, after converging on the lens A B come to a focus at m'n', where they make a real image of the arrow upside down. If you could put a piece of thin transparent paper at the point m'n' in a telescope, you would see a picture of the object upon it. The rays from this image falling on the lens C D, are again bent inwards, as by the ordinary magnifying glass (see p. 49), and thus by following them out in straight lines the eye sees a magnified arrow upside down at some point

CH. XII.

KEPLER'S THREE LAWS.

46

97

between CD and м N. Kepler's telescope is called the 'Astronomical telescope.' It has a much larger 'field of view' than Galileo's ; that is, it enables you to see over a larger space at one time; but, on the other hand, it turns

A B, Object glass. C D, Eye-piece. m n, Real arrow. m'n', Picture of the arrow formed at the focus of the rays. M N, Magnified arrow.

everything upside down. In making astronomical observations it is not of much importance which part of a star is uppermost; but for terrestrial telescopes another lens has to be put in to bring the images back to their right positions, and since Kepler's time many other improvements have been made.

Kepler's first Law, 1609.-After Tycho Brahe's death Kepler went on working at the 'Rudolphine Tables,' and this led him to consider again the movements of the planets, and to try and find a theory to explain the path or orbit of the planet Mars. Mars is the planet which stands fourth from the sun; thus Mercury is nearest to the sun, then

'This figure and also fig. 8 were kindly drawn for me by Mr. A. R. Wallace.

comes Venus, then our earth, and then outside our earth is Mars. Tycho had noted in his tables the places at which the planet had been seen at certain periods; and from these observations Kepler calculated where it ought to arrive at other fixed times if it moved in a circle, as the earlier astronomers had supposed. But he found that it did not arrive there as computed, and he was so sure that Tycho's observations were exact that he said boldly, 'All the theories must be wrong if they do not agree with what Tycho saw.' So he puzzled on, trying one explanation after another, until at last he discovered three remarkable laws, by which the movements not only of Mars, but of all the other planets, are explained.

The first of these laws is that planets move round the sun in ellipses or ovals, and not in circles. You know that to draw a circle you put one leg of the compasses into a spot and draw the other leg round it, and the middle spot is called the centre or focus. But to draw an ellipse you must have two focuses or foci. To understand this, stick two pins a little distance apart in a piece of paper, and fasten a string to them by its two ends. Place a pencil upright in the string, so as to keep it tightly stretched, and draw the pencil round first on one side then on the other. You will then have an ellipse, and the two pin-holes will be the two foci. Draw the sun in one of the foci and a round globe on some part of the ellipse, and you will have a figure of the path of our earth or any of the planets round the sun. You will find that the farther you put the pins apart the flatter the ellipse will be. The path or orbit of the planet Mercury is much more elliptical than the orbit of the Earth. Another difference in the orbits of the planets is that they do not all lie in the same direction, though they all have the sun as one of their foci. For instance in Fig. 10, the orbit