Fig. 145.—Prism Forming a Spectrum.
Fig. 146.—Convex Lens.
A concave lens is a lens which is thinner at the middle than it is at the edge, as shown in [Fig. 147]. It is used in small telescopes and opera glasses to turn the inverted picture formed by the convex lens around so that the object can be seen in its right position, as shown in [Fig. 151].
Shadows.—Shadows are useful as well as sunshine, but shadows are such common, everyday things it seems almost useless to talk about them; still you may or may not know that there are different kinds of shadows.
Fig. 147.—Concave Lens.
Of course we all know that when a candle or a gaslight or the Sun shines on an apple or any other opaque object—that is, an object that will not let the ether waves go through it—the light is cut off back of it and this dark space is called a shadow; this is also true when the Sun shines on the Earth, or on any of the other planets or their moons.
There are always two parts to the shadow of an object unless the light is a mere point or the object is very close to the screen, or surface on which the shadow falls. The dark part of the shadow is called the umbra. The edge of the object where the light and shade run together and form a partial shadow is called the penumbra and during a total eclipse this partial shadow surrounds the dark shadow, or umbra of the Earth or Moon.