332 GREAT MEN OF SCIENCE 



the observer; and the principle states that when the two ap- 

 proach one another, a shortening of the wave-length seems to 

 the observer to occur, while a lengthening takes place when 

 they recede from one another; the two effects being in a defin- 

 ite dependence upon the rate of change of the distance apart. 



The principle was first experimentally proved in connec- 

 tion with sound waves, the source of sound being situated 

 upon a locomotive in rapid motion; the pitch of the note 

 appears to the observer raised when the source of sound is ap- 

 proaching him, and lowered when it is receding from him, as 

 the principle requires. In the case of light we have to ex- 

 pect, in place of a change of pitch, a displacement in the 

 position on the spectrum, when the source of light and the 

 observer change their distance from one another. The rate 

 of change must, on account of the great velocity of light, be 

 considerable, if the displacement in the spectrum is to be 

 capable of observation. For this reason it was for the mo- 

 ment only to be expected in the case of the heavenly bodies, 

 in which great velocities frequently occur. Observations 

 capable of clear interpretation became possible as soon as 

 the lines in the spectrum of the stars had been assigned by 

 Kirchhoff and Bunsen with certainty to definite elements. 



The position of every definite line in the spectrum of a 

 certain star could then be compared with the position of the 

 same line of the corresponding element in an earthly source 

 of light; from the distance apart of the two lines the rate of 

 change of the distance between star and earth could then be 

 calculated by Doppler's principle. We already know these 

 'radial velocities' in the case of thousands of stars, which 

 means for us an entirely new possibility of discovering the 

 actual motions taking place in cosmic space. Also in the 

 case of the internal motion of certain heavenly bodies such as 

 the nebulae, spectroscopic observation of the radial velocity 

 has already led to important conclusions. 



Special mention should be made of the discovery of many 



