CHI. XXI. DOUBLE REFRACTION OF LIGHT.

FIG. 33.

A

179

the ether. Let your board be about 2 feet long, and at one end of it glue on pieces of thick-piled velvet of the shape of lenses (see 1, 2, 3, Fig. 33). Let your runner first go straight down the board upon the velvet; it will then run through the velvet without changing its course, as a vertical ray does through a lens. Then start it obliquely across the board so that it will reach the lens I in the position B. Here the left wheel of the runner will touch the velvet first, and will be checked by the rough pile, while the right wheel moves on quickly as before, and thus the runner will swing round or be refracted towards the thick part of the lens. Then, as it passes out again the right wheel will come out of the velvet first and will move more quickly on the smooth board, while the left is still checked by the velvet at c; therefore the runner will again be shifted round or refracted as it passes out. You can easily follow the course of the runner through the other lenses for yourself, always noticing that the arrow marks which way the ray of light is coming; and when you have done this you will have a beautiful imitation of the way in which the waves of light are refracted in passing through different mediums.

Figures illustrating the passage of the waves of light through differentshaped lenses (Tylor).

Double Refraction.-There is still one more remarkable

fact about light which Huyghens explained; namely, the double refraction of light through a crystal called Iceland spar. A physician of Copenhagen named Erasmus Bartholinus had received from Iceland a crystal in the form of a rhomboid (see Fig. 34), which, when broken, fell into pieces of the same shape. Bartholinus called this crystal Iceland spar,' and while making experiments with it he observed that an inkspot or any small object.

FIG. 34.

A spot of ink seen through a seen through it appeared to be doubled. crystal of Iceland spar. He was not able to explain this curious fact, but he published an account of it in 1669, and Huyghens accounted for it quite correctly by suggesting that the crystal was more elastic in one direction than in the other, so that a wave of light passing into it was divided into two waves moving at different rates through the crystals. This would cause them to be bent differently-one according to Snell's ordinary law of refraction (see p. 107), and the other in an extraordinary way. Thus these two separate rays falling upon the eye would cause there the impression of two objects.

This curious effect is very interesting to study, and it led Huyghens to make a number of remarkable experiments. He found that the two rays when they passed out at the other side of the crystal remained quite separate the one from the other, and if they were afterwards sent through another crystal in the same direction that they had gone through the first, they went on each their own way. But now came a very extraordinary fact: if the second crystal was turned round. a little so that the rays passed in rather a different direction through it, each ray was again split up into two, so that there were now four rays, sometimes all equally bright, sometimes

CH. XXI.

POLARIZATION OF LIGHT.

181

of unequal brightness, but the light of all four was never greater than the light of the one ray, out of which they had all come. These four rays continued apart while he turned the second crystal more and more round; till, when he had turned it 90°, or a quarter of a circle, the rays became two again, with this remarkable peculiarity, that they had changed characters! The ray which before had been refracted in the ordinary way now took the extraordinary direction, while the other chose the ordinary one.

This curious effect observed by Huyghens is now known as the 'polarization of light' by crystals. It is very difficult to understand, and you must be content at present to know that he discovered the fact. There is a beautiful explanation of it, but we must wait for that till we consider the science of the nineteenth century, for it is now much better understood. Huyghens' 'Theory of Light' was published in 1690, under the title Traité de la Lumière.' He remained in Paris for some years; but left it and returned to Holland when the persecution of the Protestants began after the revocation of the Edict of Nantes. He died in 1695.

Chief Works consulted.--- Herschel's 'Familiar Lectures'. - art. 'Light;' Tylor, 'On Refraction'-'Nature,' vol. ix.; 'Edin. Phil. Journal,' vols. ii. and iii.-'On Double Refraction;' Ganot's 'Physics;' Encyclopædias - Britannica,' 'Metropolitana,' and Brewster's.

CHAPTER XXII.

SUMMARY OF THE SCIENCE OF THE SEVENTEENTH

CENTURY.

We have now arrived at the close of the seventeenth century, and it only remains for us, before going farther, to try and picture to ourselves the great steps in advance which had been made between the years 1600 and 1700. We saw at p. 82 that the work of the sixteenth century consisted chiefly in making men aware of their own ignorance, and teaching them to inquire into the facts of nature, instead of merely repeating what they had heard from others. In the seventeenth century we find them following out this rule of patient research, and being rewarded by arriving at grand and true laws.

Astronomy. To begin with Astronomy. Here Galileo led the way with his telescope. The structure of the moon, with its mountains and valleys; the existence of Jupiter's four moons revolving round it and giving it light by night; the myriads of stars of the Milky Way; the spots of the sun coming into view at regular intervals, and thus proving that the sun turns on its axis; all these discoveries forced upon men's minds the truth that our little world is not the centre of everything, but a mere speck among the millions of heavenly bodies. But while they humbled man's false pride. in his own importance, they taught him on the other hand the true greatness which God has put in his power by giving

CH. XXII.

SUMMARY.

183

the intellect to discover and understand these wonderful truths if he will only seek them in an earnest and teachable spirit.

Then came Kepler with a still grander lesson, for he showed that the movements of the planets are governed by regular and fixed laws, which can be traced out so accurately that an astronomer is able to foretell with confidence what will happen many years after he himself has passed away. Thus we see Gassendi and Horrocks, by the use of Kepler's labours, calculating within a few minutes the time of a planet's passage across the face of the sun and watching the exact fulfilment of the prediction. Nor is this all : so exact and true are these movements, and so completely is man able to read them rightly, that by this simple passage of a small black spot across the sun Halley showed that we may actually number the millions of miles between ourselves and the great light around which we move. We might almost think that we had now travelled as far as man's mind could go, but something far greater remained behind. Newton sitting under his apple-tree and pondering on the wonderful mechanism of the heavens, found the one great law which accounts for the movements of all the bodies in the universe -a law which explains equally why a pin falls to the ground and why a comet which has been lost from sight for more than a hundred years will return to a certain fixed spot at a day and an hour which can be accurately foretold. Kepler had pointed out fixed and definite laws by which the universe is governed; Newton demonstrated that one law explains them all. He showed us how one single thought, as it were, of the Divine mind suffices to govern the most complicated as well as the simplest movements of our system.