5i6 THE POPULAR SCIENCE MONTHLY. 



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 conclu- 

 sion 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 dis- 

 turb it in the least." 



Among the more special questions undergoing investigation 

 at present by the application of physical principles is the deter- 

 mination of the relative motion of the heavenly bodies by spec- 

 troscopic 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 to- 

 ward 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 in- 

 creased. 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 spectrom- 

 eter 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 de- 

 termine whether the earth is approaching the star or nebula or re- 

 ceding 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 suffi- 

 ciently 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 charac- 

 ter, 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 



