May.

The Professor heard this, for he was just coming up stairs.

"What makes them twinkle, May?" and the Professor seated himself in an
easy-chair, as if ready to talk.

"I don't know, Sir. Won't you please tell us?"

"Pooh, May, don't bother the Professor with such juvenile questions.
He'll think he's intrusted with the charge of the third form in an
infant school."

"But," persisted May, "I would like very much to know, and I don't
believe you can tell, if you _have_ been to college. Now there!"

Jack was somewhat nonplussed at this, but after a moment's hesitation
said, "Well, anyhow, the books I studied never told anything about the
stars twinkling, and I don't believe they do twinkle. It's nothing but a
baby notion."

All eyes are now intently fixed upon the Professor, who is expected
first to settle the fact, and then to account for it.

"You, Jack, who have been to college," he began, "know that all vision
or sight is produced by rays falling upon the eyes. These rays may be
broken or turned from their natural course--the word astronomers use is
refracted. Now the stars are so far away from us that through the
largest telescope they are still only points of light. As the rays come
down through space there is nothing to 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