Page:Astronomy for Everybody.djvu/97

The spectroscope is an instrument for analysing light. It is a much more recent instrument than the telescope, having first been applied to astronomical observation about 1864. To convey an intelligent idea of its use we must say something about the heat and light radiated by the heavenly bodies.

We know that the sun, a gas light, or other bright body gives us heat as well as light. A very simple observation will show that the rays of heat proceed in straight lines like those of light, and that they can pass through air and other transparent bodies without warming them, just as light does. If we make a large fire on the hearth in a perfectly cold room, we shall feel the heat on our faces although the air may be frosty. A striking experiment is that of making a lens out of ice and using it as a burning glass. The rays of the sun passing through the ice may be concentrated so as to burn the hand, and that without the ice melting.

It was formerly supposed that heat and light were two distinct agents; now it is known that such is not the case. As emitted by a hot body both may be called by the general name of radiance. All radiance, when it falls on a surface, produces heat, just as the blaze of the fire produces heat on the walls of a room. But not all radi-