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CH. IX.

PARACELSUS AND VAN HELMONT.

73

though he did not describe them clearly, and he particularly mentions the gas which rises from beer and other liquids which ferment. He called this Gas sylvestre. The chief thing to remember about Van Helmont is that he was the first writer to use the word 'gas,' which he took from the German word 'geist,' meaning 'spirit.'

Chief Works consulted.-Rees's 'Encyclopædia,' art. 'Copernicus;' Encyclopædia Metropolitana,' art. 'Astronomy;' 'Biographie Universelle,' art. 'Copernicus;' Gassendi's Life of Copernicus ;' 'Encyclopædia,' art. 'Anatomy;' Cuvier, 'Histoire des Sciences Naturelles,' 1845; D'Orbigny, 'Dict. des Sciences Naturelles'--Introduction; Encyclopædia,' art. 'Botany;' Hoefer, 'Histoire de la Physique et de la Chimie,' 1850,

CHAPTER X.

SCIENCE OF THE SIXTEENTH CENTURY (CONTINUED).

Baptiste Porta discovers the Camera Obscura-Shows that our Eye is like a Camera Obscura-Makes a kind of Magic Lantern by Sunlight-Kircher afterwards makes a Magic Lantern by Lamplight -Dr. Gilbert's discoveries in Electricity-Tycho Brahe, the Danish Astronomer-Builds an Observatory on the Island of Huen-Makes a great number of Observations, and draws up the Rudolphine Tables-Galileo discovers the principle of the Pendulum-Calculates the velocity of Falling Bodies, and shows why it increases Shows that Unequal Weights fall to the Ground in the same time-Establishes the relations of Force and Weight-Summary of the Science of the sixteenth century.

Baptiste Porta's discoveries about Light, 1560.—The next discovery in science was about Light, and it was made by a boy only fifteen years of age. Baptiste Porta was born in Naples in 1545. He was so eager for new knowledge that when quite a boy he held meetings in his house for any of his friends to read papers about new experiments. These meetings were called 'the Academy of Secrets,' and in the year 1560, when Porta was fifteen, he published an account of them in a book called 'Magia Naturalis,' or 'Natural Magic.' In the seventeenth chapter of this book he relates the following experiment which he had made himself.

He says he found that by going into a darkened room when the sun was shining brightly, and making a very small hole in the window-shutter, he could produce on the wall of

CH. X. 'CAMERA OBSCURA' AND MAGIC LANTERN. 75

the room, opposite the hole, images of things outside the window. These images were exactly the shape of the real objects, and had always their proper colours; as for example, if a man was standing against a tree outside the house, the green leaves of the tree and the different colours of the man's clothes would be clearly shown on the wall. There was only one peculiarity about the picture, it was always upside down, so that the man stood on his head, or the tree with its trunk in the air. The smaller the hole was, the clearer

were the outline and the colours of the image, and Porta found that by putting a convex lens (that is, a glass with its surfaces bulging in the centre, see p. 49) into the hole he could get a still brighter and clearer picture at a particular point in the room.

Porta knew from the works of Alhazen that rays of light are reflected in all directions from every object, and he explained this image on the wall quite correctly, by saying that the small hole lets in only one ray from each point of an object outside; the other rays and those from the sky and other objects being kept out by the shutter. Thus these single rays fall directly on the wall without being mingled with others, and so make a clear picture. It is easy to see from fig. 7 that the image must be upside down, because the rays cross in going through the hole. This simple discovery of Porta's is called the 'Camera Obscura,' or 'Dark Chamber.'

You may perhaps have been into one at the sea-side, where they build them for visitors to watch the coloured reflection of the passers-by. In the camera obscura, as it is now made, the glasses are so arranged that the figures are upright.

Porta saw at once how useful this invention would be for making accurate drawings of objects; for, by tracing out with colours on the wall the figure of the man or tree as it stood, he could get a small image of it with all its proportions and colours correct. But, what is still more important, he was led by this experiment to understand how we see objects, and to prove that Alhazen was right in saying that rays of light from the things around us strike upon our eye. For, said Porta, the little hole in the shutter with the lens in it, is like the little hole in our eye, which also contains a natural convex lens; and we see objects clearly because the rays pass through this small hole. He did not, however, know which part of our eye represents the wall on which the figure is thrown, nor why we see objects upright; we shall see (p.96) that Kepler discovered this many years afterwards.

When Porta had succeeded in getting clear images of real things on the wall, he began to try painting artificial pictures on thin transparent paper and passing them across the hole in the shutter, and he found that the sun threw a very fair picture of them on the wall. In this way he produced representations of battles and hunts, and so made a step towards the Magic Lantern. He seems, however, never to have tried it by lamplight; this was done by Kircher, a German, about fifty years later. There is no doubt that Porta had a very good notion of how to use two magnifying glasses so as

CH. X.

DISCOVERY OF ELECTRICITY.

77

to make objects appear nearer and larger, but it is not certain that he ever really made a telescope.

Dr. Gilbert, the Founder of the Science of Electricity, 1540-1603.-It was about this time, while Baptiste Porta was making experiments on light in Italy, that an Englishman named Gilbert made the first step in one of the most wonderful and interesting of all the sciences, namely, that of Electricity. So long ago as the time of the Greeks it was already known that amber, when rubbed, will attract or draw towards it bits of straw and other light bodies, and it is from the Greek word electron-amber, that our word electricity is taken.

Until the sixteenth century, however, no one had made any careful experiments upon this curious fact, and it was Dr. Gilbert, a physician of Colchester, who first discovered that other bodies besides amber will, when rubbed, attract straws, thin shavings of metals, and other substances. You can easily try this for yourself by rubbing the end of a stick. of common sealing-wax on a piece of dry flannel, and then holding the rubbed end near to some small pieces of light paper, or some feathers or bran. You will find that these substances will spring towards the sealing-wax and cling to it for short time, being held there by the electricity which has been produced by rubbing the sealing-wax.

Gilbert showed that amber, jet, diamond, crystal, sulphur, sealing-wax, alum, and many other substances, have this power of attraction when they are rubbed, and he also proved that the attraction was stronger when the air is dry and cold than when it is warm and moist. This may seem very little to have discovered compared to the wonderful facts which we now know about electricity; but it was the first step, and Gilbert's book on 'Magnetism' (as he called