FIG. 187.—Comparison of the Absorption Spectrum of the Sun with the Radiation Spectra of Iron and Calcium, with Common Impurities. ] There is also another quite distinct line of inquiry in which the spectroscope helps us. Imagine yourself in a ship at anchor, and the waves passing you, you can count the number per minute; now let the vessel move in the direction whence the waves come, you would then meet more waves per minute than before; and if the vessel goes the other way, less will pass you, and by counting the increase or decrease in the number passing, you might estimate the rates at which you were moving. Again, suppose some moving object causes ripples on some smooth water, and you count the number per minute reaching you, then if that object approach you, still moving, and so producing waves at the same rate, the number of ripples a minute will increase, and they will be of course closer together; for as the object is approaching you, every subsequent ripple is started, not from the same place as the preceding one, but a little nearer to you, and also nearer to the one preceding, on whose heels it will follow closer. By the increase in the number of ripples, and also the decrease in the distance between them, one can estimate the rate of motion of the object producing them, for the decrease in distance between the ripples is just the distance the object travels in the time occupied between the production of two waves, which was ascertained when the object was stationary. Now let us apply this reasoning to light. Light, we now hold, is due to a state of vibration of the particles of an invisible ether, or extremely rare fluid, pervading all space; and the waves of light, although infinitesimally small, move among these particles. Now we know that it is the length of the waves of light which determines their refrangibility or colour, and therefore anything that increases or diminishes their length alters their place in the spectrum; and as waves of water are altered by the body producing them moving to or from the observer, so the waves of light are changed by the motion of the luminous body; and this change of refrangibility is detected with the spectroscope. By measuring the wave-length of let us say the F line, and the new wave-length as shown by the changed position, we can estimate the velocity at which the light source is approaching or receding from us.