DRIFTING LIGHT-WAVES. 103 



Airy, " to avoid any bias from previous knowledge of the 

 direction in which a displacement" (of the spectral lines) 

 "was to be expected," the side of the sun under obser- 

 vation not being known by the observer until after the 

 observation was completed. 



But Professor Young, at Dartmouth College, Hanover, 

 N.H., has done much more than merely obtain evidence by 

 the new method that the sun is rotating as we already knew. 

 He has succeeded so perfectly in mastering the instru- 

 mental and observational difficulties, as absolutely to be 

 able to rely on his measurement (as distinguished from the 

 mere recognition) of the sun's motion of rotation. The 

 manner in which he has extended the powers of ordinary 

 spectroscopic analysis, cannot very readily be described in 

 these pages, simply because the principles on which the 

 extension depends require for their complete description 

 a reference to mathematical considerations of some com- 

 plexity. Let it be simply noted that what is called the 

 diffraction spectrum, obtained by using a finely lined plate, 

 results from the dispersive action of such a plate, or grating 

 as it is technically called, and this dispersive power can be 

 readily combined with that of a spectroscope of the ordin- 

 ary kind. Now Dr. Rutherfurd, of New York, has succeeded 

 in ruling so many thousand lines on glass within the breadth 

 of a single inch as to produce a grating of high dispersive 

 power. Availing himself of this beautiful extension of 

 spectroscopic powers, Professor Young has succeeded in 

 recognizing effects of much smaller motions of recession 

 and approach than had before been observable by the 

 new method. He has thus been able to measure the 

 rotation-rate of the sun's equatorial regions. His result 

 exceeds considerably that inferred from the telescopic obser- 

 vation of the solar spots. For whereas from the motion of 

 the spots a rotation-rate of about \\ mile per second has 

 been calculated for the sun's equator, Professor Young 

 obtains from his spectroscopic observations a rate of rather 

 more than if mile, or about 300 yards per second more 

 than the telescopic rate. 



