of an approaching locomotive gives one tone; it passes, and we instantly detect another.  Let two trains, running at a speed of thirty-six yards a second, approach each other.  Let the whistle of one sound the note E, three hundred and twenty-three vibrations per second.  It will be heard on the other as the note G, three hundred and eighty-eight vibrations per second; for the speed of each train crowds the vibrations into one-tenth less room, adding 32+ vibrations per second, making three hundred and eighty-eight in all.  The trains pass.  The vibrations are put into one-tenth more space by the whistle making them, and the other train allows only nine-tenths of what there are to overtake the ear.  Each subtracts 32+ vibrations from three hundred and twenty-three, leaving only two hundred and fifty-eight, which is the note C. Yet the note E was constantly uttered.

[Illustration:  1.  Solar Spectrum. 2.  Spectrum of Potassium. 3.  Spectrum of Sodium. 4.  Spectrum of Strontium. 5.  Spectrum of Calcium. 6.  Spectrum of Barium.]

If a source of light approach or depart, it will have a similar effect on the light waves.  How shall we detect it?  If a star approach us, it puts a greater number of waves into an inch, and shortens their length.  If it recedes, it increases the length of the wave—­puts a less number into an inch.  If a body giving only the number of vibrations we call green were to approach sufficiently fast, it would crowd in vibrations enough to appear what we call blue, indigo, or even violet, according to its speed.  If it receded sufficiently fast, it would leave behind it only vibrations enough to fill up [Page 53] the space with what we call yellow, orange, or red, according to its speed; yet it would be green, and green only, all the time.  But how detect the change?  If red waves are shortened they become orange in color; and from below the red other rays, too far apart to be seen by the eye, being shortened, become visible as red, and we cannot know that anything has taken place.  So, if a star recedes fast enough, violet vibrations being lengthened become indigo; and from above the violet other rays, too short to be seen, become lengthened into visible violet, and we can detect no movement of the colors.  The dark lines of the spectrum are the cutting out of rays of definite wave-lengths.  If the color spectrum moves away, they move with it, and away from their proper place in the ordinary spectrum.  If, then, we find them toward the red end, the star is receding; if toward the violet end, it is approaching.  Turn the instrument on the centre of the sun.  The dark lines take their appropriate place, and are recognized on the ruled scale.  Turn it on one edge, that is approaching us one and a quarter miles a second by the revolution of the sun on its axis, the spectral lines move toward the violet end; turn the spectroscope toward the other edge of the sun, it is receding from us one and a quarter miles a second by reason of the axial revolution, and the spectral lines move toward the red end.  Turn it near the spots, and it reveals the mighty up-rush in one place and the down-rush in another of one hundred miles a second.  We speak of it as an easy matter, but it is a problem of the greatest delicacy, almost defying the mind of man to read the movements of matter.