absorbed as it is called, by the material of the leaf, and only the green waves bound back upon your eye. In other words, the vibrations of the ether coming from the leaf move exactly fast enough to produce upon your eye the sensation you call green, just as the vibration of the air caused by a particular string of a harp produces on your ear the sensation of the note you call the middle C.

Refraction of Light explained by Huyghens.-But we must now go back to Huyghens, and point out how beautifully he explained by his undulatory theory the refraction or bending-back of rays of which we have already spoken so much. When a wave of light is travelling onwards, he said, if it passes vertically into glass or any denser substance, the wave will move more slowly, but it will still go straight on, because both ends of the wave will be equally checked. But if the wave goes into the glass obliquely (see p. 47), one end of it will reach the glass first before the other, and will move slowly, while the other end goes on unchecked, and so the wave will swing round and will have its direction altered. In the same way, when it passes out again from the glass, one end will pass out first, and will move more easily in the air than the end that is still in the glass, and so it will swing round again and make another bend.

You must not be disappointed if you do not understand this at once, for it is very difficult; to make it easier we will borrow a very ingenious illustration given last year (Jan. 1, 1874) by Mr. E. B. Tylor, in a periodical called 'Nature.' Take two small wheels about 2 inches round, and mount them loosely upon a stout iron axle measuring about half-aninch round. This will make a runner like two wheels of a cart, and if you let it roll down a smooth board it will represent very fairly the crests or tops of the waves of light in

CH XXI.

DOUBLE REFRACTION of light.

179

FIG. 33.

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

FIG. 34.

A spot of ink seen through a crystal of Iceland spar.

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 seen through it appeared to be doubled. 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.

art.

Chief Works consulted. -- Herschel's 'Familiar Lectures' '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