break or refract them, but when they reach our atmosphere, there is the tremulous agitation of the air and ascending vapors. By these the rays coming from the tiny points are at intervals turned aside from the narrow space of the pupil of the eye. When the eye is assisted with the wide opening of the object-glass of a telescope no such thing happens. So Jack is right; the stars don't twinkle. When viewed through a telescope, they are found to shine with a steady brightness, and hence the motion is only in appearance. Recent astronomers have little to say about it; but it is due, doubtless, as Sir Isaac Newton explains in his celebrated Principia, to the ascending vapors and tremulous movements of the atmosphere. You have seen how the heated air or gas rising from a stove will sometimes make things behind it tremble and dance. Now if a small candle were burning on the other side of the ascending vapor, its flame, though really steady, would seem to flicker."

"Then, Sir, the stars, being so very far off, appear so very little, and the rays of light they send are disturbed by atmospheric vapor, and thus to the naked eye they twinkle."

"Yes," said the Professor. "The sun and moon, as you know, present broad disks, with light radiating from every point—"

"Please, Professor," interrupted May, "tell us what a 'disk' is. Jack's big words are dreadful to understand; and this, although a little one, seems just as bad."

"I don't wonder I puzzled you, May. We use the word disk for the face or surface of a heavenly body which appears to have some size. You may always stop me when I use a word you don't understand; but when I have once told you, I shall want you to remember; for we can not know much about science unless we learn some of the hard words. I was saying that the sun and moon present broad disks, so that if some of the light is intercepted, the eye does not notice the loss. The same is true also of the planets, which appear large when they are magnified, but not of the stars, owing to their immense distances; and when the impurities in the atmosphere obscure or divert the narrow line of light they send to us, the eye perceives it at once. Some of the stars appear very brilliant through the large telescopes, but the light still seems to proceed from a single point. There are some four or five thousand stars that can be seen without a telescope."

THE GREAT EQUATORIAL, UNITED STATES NATIONAL OBSERVATORY.

"Why," interrupted Joe, "I thought there were more than anybody could count."

"So there are," replied the Professor, "but the number that can be distinctly perceived by the unassisted eye is found to be comparatively small when they are carefully looked after. On very clear nights the whole sky may seem to glisten when the eye is suddenly turned upward; and there are some portions of it where a confused light comes from a sort of star-cloud, which has received the name of 'Milky Way.' But the stars that can be seen separately are very easily counted. Some persons can see rather more than others, on account of their eye-sight being naturally better, or improved by use. A rough count of the number that could be seen through Herschel's famous telescope made it twenty thousand. The great telescopes more recently made would probably show as many as forty or fifty millions."

"I should think," said May, "that it would be awful tiresome to count so many things just alike, and that the man would often count the same one over and over without knowing it, and would never be sure that he had counted right."

"They are not all alike," said the Professor. "They differ greatly in brightness, and to some extent in color, and in other particulars. They have been divided according to their brilliancy into sixteen classes or magnitudes. The fifteen brightest stars are said to be of the first magnitude, the fifty next of the second, and so on to the sixth, which comprises the faintest stars visible to the unassisted eye. The brightest star of all visible in our latitude is the dogstar, which gives four times as much light as any other. In every age of the world there have been learned men interested enough in the stars to make catalogues of them, giving their magnitudes and their positions."

"I think they must have been very slow and stupid follows," said Gus, "or they would have found something better to do."

At this the Professor laughed.

"I think, Gus, you are hungry, and have your mind on mutton-chops. I shall not talk to you any more this morning; but, after lunch, if you will look in one of the great books in papa's library, which he will point out, you will find pictures of all the great telescopes in the world. The best one in our own country is that at the United States National Observatory at Washington. Without the aid of these wonderful instruments we should never have learned much about the stars."

THE HISTORY OF PHOTOGEN AND NYCTERIS.

A Day and Night Mährchen.

By GEORGE MACDONALD.

I.—WATHO.

There was once a witch who desired to know everything. But the wiser a witch is, the harder she knocks her head against the wall when she comes to it. Her name was Watho, and she had a wolf in her mind. She cared for nothing in itself—only for knowing it. She was not naturally cruel, but the wolf had made her cruel.

She was tall and graceful, with a white skin, red hair, and black eyes, which had a red fire in them. She was straight and strong, but now and then would fall bent together, shudder, and sit for a moment with her head turned over her shoulder, as if the wolf had got out of her mind on to her back.

II.—AURORA.

This witch got two ladies to visit her. One of them belonged to the court, and her husband had been sent on a far and difficult embassy. The other was a young widow whose husband had lately died, and who had since lost her sight. Watho lodged them in different parts of her castle, and they did not know of each other's existence.

The castle stood on the side of a hill sloping gently down into a narrow valley, in which was a river, with a pebbly channel and a continual song.

The garden went down to the bank of the river, inclosed by high walls, which crossed the river, and there stopped. Each wall had a double row of battlements, and between the rows was a narrow walk.

In the topmost story of the castle the Lady Aurora occupied a spacious apartment of several large rooms looking southward. The windows projected oriel-wise over the garden below, and there was a splendid view from them both up and down and across the river. The opposite side of the valley was steep, but not very high. Far away snow peaks were visible. These rooms Aurora seldom left, but their airy spaces, the brilliant landscape and sky, the plentiful sunlight, the musical instruments, books, pictures, curiosities, with the company of Watho, who made herself charming, precluded all dullness. She had venison and feathered game to eat, milk and pale sunny sparkling wine to drink.

She had hair of the yellow gold, waved and rippled; her skin was fair, not white like Watho's, and her eyes were of the blue of the heavens when bluest; her features were delicate but strong, her mouth large and finely curved, and haunted with smiles.

III.—VESPER.

Behind the castle the hill rose abruptly; the northeastern tower, indeed, was in contact with the rock, and communicated with the interior of it. For in the rock was a series of chambers, known only to Watho and the one servant whom she trusted, called Falca. Some former owner had