CH. XX.

THE ACHROMATIC TELESCOPE.

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the artificial colours are not pure, and also because it is difficult to paint each colour in the proper proportion.

But now that we have proved that light is broken up into colours in passing through a denser medium, you may perhaps ask how it is that we do not see coloured rays whenever we look at the sun through glass or any other transparent substance. The reason is that when the two sides of the glass are parallel (that is, lie always at the same distance from each other), the ray of light is bent just as much in going out from the glass into the air as it was when it came in from the air into the glass, and so it remains just as it was at first. When the two sides are not parallel, as in a rounded lens, colours do appear in the thin edges of the glass, and these used to be very troublesome in telescopes and microscopes. Newton thought that they could never be got rid of, for he did not know that light is spread out or dispersed more in one kind of glass than in another. But two years after his death, in 1729, Mr. Chester More Hall, of Essex, found that two kinds of glass (flint-glass and crown-glass) disperse light differently, so that when you put them together they correct each other, and the coloured rays at the edges are blended into white light. Telescopes and microscopes which are made in this way are called achromatic (from a, without; chroma, colour). A patent for such instruments was taken out by a Mr. Dollond in 1757, and he probably invented them without having heard of Mr. Hall's discovery.

It would require a whole volume to give you all Newton's investigations into the nature of light, and his experiments on the coloured rings of the soap-bubble and other transparent substances. His work on Optics was read before the Royal Society in 1671 and 1672, but the ideas were so new that many clever men, who should have known better,

attacked him with a number of foolish and ignorant objections, till at last he told his friend Huyghens that he was almost sorry he had ever made them public.

After his great work, the 'Principia,' had been published in 1687, he next turned his attention to chemistry, but unfortunately all the results of his labour in this science were destroyed by an accident. One day when he was in chapel, his pet dog Diamond turned over a lighted taper, which set fire to all the papers on which his work was written. When he returned and found the charred heap it is said that he merely exclaimed, 'Oh Diamond, Diamond! thou little thinkest the mischief thou hast done!' but his grief at the loss of his work affected his brain, and though he recovered and lived another forty years, publishing many editions of his works, yet he never made any more great discoveries.

Newton received many honours in his old age in 1699 he was elected Master of the Mint, and a member of the French Royal Academy of Sciences; in 1703 he was made President of the Royal Society, and in 1705 he was knighted by Queen Anne. Like all truly great men, he was modest as to his own abilities, and always willing to be taught by others. He felt so strongly how much we have still to learn about the Universe, that he considered his own discoveries as very trifling indeed. A short time before his death he said of himself, 'I know not what the world may think of my labours; but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.' Yet this man who spoke so humbly was the discoverer of the greatest and most universal law

CH. XX.

DEATH OF NEWTON.

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known to mankind! He loved to seek out new laws, but he was more anxious to collect facts and to make sure that he was right, than eager to publish his conclusions. It was the truth he loved, and not the fame which it brought. His patience and perseverance were unbounded; he was never in a hurry, but turned a subject over and over in his mind for years together, seizing upon every new light shed upon it, and waiting patiently for more. And through all his labours he looked reverently up to the One Great Light whose guiding power he loved to trace and to acknowledge in all the wonders of the universe. He died in 1727 at eighty-five years of age, and was buried in Westminster Abbey, his pall being borne by the first nobles of the land.

Chief Works consulted. Newton's 'Optics,' 1721; Ganot's 'Physics;' Rossiter's 'Physics;' Brewster's 'Encyclopædia,' art. 'Optics; Herschel's 'Familiar Lectures.'

CHAPTER XXI.

SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). Roemer measures the Velocity of Light-Newton's Corpuscular Theory of Light-Undulatory or Wave-theory proposed by Huyghens Invention of Cycloidal Pendulums by Huyghens-Discovery of Saturn's Ring-Sound caused by Vibration of Air-Light by Vibration of Ether-Reasons why we see Light-Reflection of Waves of Light-Cause of Colour-Refraction explained by the Undulatory Theory-Mr. Tylor's Illustration of Refraction-Double Refraction explained by Huyghens - Polarisation of Light not understood till the nineteenth century.

Olaus Roemer measures the Velocity of Light, 1676.— While Newton was dispersing light in prisms, and finding out its nature, Olaus Roemer, a famous Danish astronomer (born 1644, died 1710), was engaged in something almost as wonderful. He was measuring the rate at which light travels across the sky! It seems at first as if this would be impossible, but we now know three different ways of accomplishing it; Roemer's was the first attempt ever made, and his measurement was very near indeed to the truth.

You will remember that Jupiter has four moons, which move round it as our moon moves round our earth. Three of these moons are so near Jupiter and move round it in such a manner that they pass through its shadow and are eclipsed every time they go round. Now it became very

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VELOCITY OF LIGHT.

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useful, for certain astronomical reasons, to know exactly when these eclipses happened, and the time of their occurrence was therefore calculated very carefully ever since Galileo first discovered them. There was no difficulty in doing this, and yet, strange to say, the eclipses rarely happened exactly at the right moment. Sometimes they were too early, sometimes too late, and they varied according to some regular rule as much as 16 minutes 36 seconds on each side of the exact moment when they ought to have happened.

At last it occurred to Roemer, and to an Italian astronomer named Cassini, that, as Jupiter is farther away from the earth at one time than at another, the eclipses might be seen some minutes later whenever the rays of light from the moons had to cross a greater distance to reach the earth. Cassini seems to have put the thought aside and not to have worked it out; but Roemer seized upon it, and by careful calculations proved that it was the true answer to the difficulty. If the earth was at E (Fig. 32) for example, when Jupiter

FIG. 32.

E

E

Different Distances at which Jupiter's Light reaches the Earth.

J, Jupiter. E E', The earth.

was at J, the light would not have nearly so far to travel as if the earth was at E'; and in this last position the 16 minutes 36 seconds would be taken up by the light crossing the earth's orbit from E to E'. This distance was known to be about 190,000,000 miles, so that light travels at the rate