To improve the seeing qualities of the pinhole telescope make the hole in the cardboard disk ¼ inch in diameter and cover this hole with a bit of tinfoil. Now make a hole in the center of the tinfoil with the point of a needle; this makes the edge of the hole sharp.
A pasteboard tube without the pinhole will also aid the naked eye in seeing the Moon and stars, for it shuts out all the rays of light around the eye and limits the sight to a certain part of the sky; together these things are very good helps in observing especially if there are gas and electric lights nearby.
Fig. 151.—The Telescope (Galileo).
How a Telescope Works.—A real telescope has at least two lenses in it; the larger lens is placed in the end of the tube nearest the object to be viewed and is called the object glass because the light from the object is received by it.
The smaller lens is placed in the end of the tube nearest the eye, and is called the eyepiece, for it is this lens which enlarges, or magnifies the image of the object that is thrown upon the retina of the eye.
There are two simple kinds of telescopes; the first is the kind that Galileo used for making his great discoveries. The kind of lenses used and the way they are placed in the tube is shown in [Fig. 151].
In this telescope the object glass is a double convex lens and the beam of light which strikes it is brought to a point as in the case of a burning glass, but before the light reaches this point it is caught up by the double concave lens which forms the eyepiece, when it is carried to the eye in an erect position.
Fig. 152.—Opera Glasses.