Page:Popular Science Monthly Volume 45.djvu/534

516 the earth. He says: "I do not believe the ether moves. It does not move at a five-hundredth part of the speed of the steel disks" (used in the experiment). "I hope to go further, but my conclusion so far is that such things as circular saws, fly wheels, railway trains, and all ordinary masses of matter do not appreciably carry the ether with them. Their motion does not seem to disturb it in the least."

Among the more special questions undergoing investigation at present by the application of physical principles is the determination of the relative motion of the heavenly bodies by spectroscopic methods. It is done by applying to light-waves what is known in acoustics as Doppler's principle. The position of any line of the spectrum depends upon the wave length, or, what comes to the same thing in this case, the period of vibration for the particular set of waves making the light at that line in the spectrum. By increasing the number of waves per second that fall upon the prism (or grating) of the spectrometer, the period is correspondingly decreased, and conversely. Therefore, while the rate of vibration remains constant, if the grating is moving toward the source of vibration, the number of waves per second falling upon the grating will be greater, and their period smaller, than if the source and the grating are stationary relatively to each other. If they are separating, the period of vibration is increased. In the former case the line of the spectrum will be more refracted, in the latter less refracted, than in a normal case. When a spectrum line of any of the heavenly bodies has been identified with that of any substance known to us, the spectrometer gives the means of determining the motion of such heavenly bodies as compared with the motion of the earth, by observing the displacement of the spectrum line. That is, it is possible to determine whether the earth is approaching the star or nebula or receding from it, and at what rate. This method was proposed and attempts were made to apply it very early in the history of the spectroscope, but the means of observation were not then sufficiently fine, and only negative results were obtained. Within the last few years, however. Prof. Huggins, Prof. Vogel, and others in Europe have made many successful measurements of this character, and Prof. Keeler, of the Alleghany Observatory, has greatly extended them. These relative motions are usually reduced to the sun, the results indicating the relative motion of the sun and the heavenly body observed. As instances. Prof. Keeler finds that the great nebula in Orion is receding from the sun at the rate of eleven miles per second; and by observations between April and August, 1890, the sun was at that time approaching the bright star Arcturus at the rate of four miles and three tenths per second. These serve as a fine illustration of modern methods