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

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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. 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.

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

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

CH. VIII. FLAVIO GIOJA-MARINER'S COMPASS.

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people will make machines to fly in the air. This shows that he must have imagined many things which were not really discovered till more than 300 years afterwards; but they were all dreams which he could not carry out hintself. Before we leave Roger Bacon I must warn you not to confuse him with Francis Bacon, Chancellor of England, who was quite a different man, and lived more than 200 years later.

Flavio Gioja discovers the Mariner's Compass, 1300.— About ten years after the death of Bacon, a man was born in a little village called Amalfi, near Naples, who made a discovery of great value. The man's name was Flavio Gioja, and the discovery was that of the mariner's compass. Long before Flavio's time people knew that there was a kind of stone to be found in the earth which attracted iron. There is an old story that this stone was first discovered by a shepherd, who, while resting, laid down his iron shepherd's crook by his side on a hill, and when he tried to lift it again it stuck to the rock. Although this story is probably only a legend, yet it is certain that the Greeks and most of the ancient nations knew that the loadstone attracted iron; and a piece of loadstone is called a magnet, from the Greek word magnes, because it was supposed to have been first found at Magnesia, in Ionia.

A piece of iron rubbed on a loadstone becomes itself a magnet, and will attract other pieces of iron. But Flavio Gioja discovered a new peculiarity in a piece of magnetised iron, which led to his making the mariner's compass. He found that if a needle or piece of iron which has been magnetised is hung by its middle from a piece of light string, it will always turn so that one end points to the north and the other to the south. He therefore took a