He knew that the air round our globe grows denser as it gets nearer the earth, so he argued that the slanting rays from the sun, moon, and stars must become bent as they approach the earth and pass through the denser air. This, he said, causes us to see the sun after it has really sunk below our horizon at night, and before it rises in the morning; for the rays are gradually curved by passing through the denser air round our earth. Fig. 4 explains this.

s, Sun.

Earth

Bending of the Sun's rays by the atmosphere.

SC and SD, Rays as they would travel if there were no atmosphere. SBA, Ray bent so that the sun becomes visible at A.

way;

Supposing the sun to be at s, and a person at a, it is clear that any straight ray from the sun, such as s D, could not reach A, because part of the earth is in the neither could a ray, s c, reach the earth, because it would pass . above it. But when the rays from s to c strike the atmosphere at B, they are bent out of their course, and are gradually curved more and more by the denser air till they are brought down to the earth at A, and so the sun becomes visible.

Alhazen was also the first to remark that a convex lens, that is, a glass with rounded surfaces, such as our common magnifying glasses and burning glasses, will make things appear larger if held at a proper distance between the eve

CH. VII.

ALHAZEN-MAGNIFYING GLASSES.

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and any object, because the two surfaces of the glass, becoming more and more oblique to each other as they approach the sides, bend the rays inwards, so that they come

FIG. 5.

B

Arrow magnified by a convex lens.

to a focus in the eye. To understand this, draw a line of any kind, say a little arrow, on a sheet of paper, and bring your eye near to it. Your arrow being so close would look very large if you could see it distinctly, but just because it is so near, your eye cannot focus or collect together the rays coming from it so as to make a picture on the retina at the back of the eye; therefore you see nothing but an indistinct blur. But now take a magnifying glass, C D, fig. 5, and hold it between your eye and the arrow. If you hold it at the right distance you will now see the arrow distinctly, because the greater part of the rays have been bent or refracted by the rounded glass so as to come into focus on your retina. But now comes another curious fact. It is a law of sight, that when rays of light enter our eye we follow them out in straight lines, however much they may have been bent in coming to the eye. So your arrow will not appear to you as if it were at a b, but, following out the dotted lines, you will see a magnified arrow, A B, at the

distance at which you usually see small objects distinctly. This observation of Alhazen's about the bending inwards or converging of rays through rounded glasses was the first step towards spectacles.

Besides the Arabians whom I have mentioned here, there were many who were very celebrated, but we know very little of their works. Among them was Avicenna A.D. 980, whom you will often hear mentioned as a writer on minerals. But the chief thing to be remembered, besides the discoveries of Geber and Alhazen, and the introduction of the Indian numerals, is that in the Dark Ages, when all Europe seemed to care only for wars and idle disputes, it was the Arabs who kept the lamp of knowledge alight and patiently led the way to modern discoveries.

CH. VIII.

ROGER BACON.

CHAPTER VIII.

SCIENCE OF THE MIDDLE AGES (CONTINUED).

Roger Bacon-His ‘Opus Majus'—His Explanation of the Rainbow He makes Gunpowder-Studies Gases-Proves a Candle will not burn without Air-His Description of a Telescope-Speaks of Ships going without Sails-Flavio Gioja invents the Mariner's Compass-Greeks knew of the Power of the Loadstone to attract Iron - Use of the Compass in discovering new lands - Invention of Printing-Columbus discovers America-Vasco de Gama sees the Stars of the Southern Hemisphere-Magellan's ship sails round the World-Inventions of Leonardo da Vinci.

We must now return to Europe, where the nations were struggling out of the Dark Ages; and though there were many learned men in the monasteries, very few of them paid any attention to science: while those who did, often lost their time in alchemy, trying to make gold; or in astrology, pretending to foretell events by the stars.

Roger Bacon, 1214:-In the year 1214, however, a man was born in England whom every Englishman ought to admire and revere, because in those benighted times he gave up his whole life to the study of the works of nature, and suffered imprisonment in the cause of science. This was Roger Bacon, a great alchemist, who was born at Ilchester in Somersetshire, educated at Oxford and Paris, and then became a friar of the order of St. Francis. For this reason

he is often called Friar Bacon. Bacon's great work, called

the Opus Majus,' is written in such strange language that it

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is often difficult to find out how much he really knew and how much he only guessed at. We know, however, that he made many good astronomical observations, and that he explained the rainbow by saying that the sun's rays are refracted or bent back by the falling drops of rain, as was also noticed about the same time by Vitellio, a Polish philosopher.

Bacon is famous as the first man in Europe who made gunpowder; we do not know whether he learnt the method from the Arabs, but it is most likely, for he gives the same receipt for making it as Marcus Græcus did-namely, saltpetre, charcoal, and sulphur. He also knew that there are different kinds of gas, or air as he calls it, and he tells us that one of these puts out a flame. He invented the favourite schoolboy's experiment of burning a candle under a bell-glass to prove that when the air is exhausted the candle goes out.

Bacon seems also to have known the theory of a telescope. We do not know whether he ever made one, but he certainly understood how valuable it would be. This is what he says about it in his 'Opus Majus,' or 'Great Work': 'We can place transparent bodies (that is, glasses) in such a form and position between our eyes and other objects that the rays shall be refracted and bent towards any place we please, so that we shall see the object near at hand, or at a distance, under any angle we please; and thus from an incredible distance we may read the smallest letter, and may number the smallest particles of sand, by reason of the greatness of the angle under which they appear.' This is at least a very fair description of a telescope. In the same book he says that one day ships will go on the water without sails, and carriages run on the roads without horses, and that