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."

"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 bee