272 PHYSICAL SCIENCE 



detected in any reasonable time. If, however, 

 the star is moving towards or away from the 

 earth, the spectroscope is turned towards it, and 

 in the short time required to fix a photographic 

 impression, develop and print the plate and 

 measure the lines upon it, the velocity of the 

 star can be determined. 



Another application of the same principle has 

 enabled us to demonstrate directly the rotation 

 of the sun on its axis, and to separate those 

 absorption lines in the spectrum of the sun's 

 light which are due to the effect of the earth's 

 atmosphere from the lines of true solar origin. 

 One limb of the sun is, at any moment, approach- 

 ing the earth, while the opposite limb is in like 

 manner receding. By pointing a spectroscope 

 first at one limb and then at the other, a shift 

 of the spectral lines is seen ; and, from the amount 

 of the displacement, the velocity of movement of 

 the glowing gases which produce the lines of 

 absorption can be calculated. Lines which are 

 not shifted by this operation are clearly not of 

 solar origin, and are consequently to be referred 

 to absorption by the atmosphere of the earth. 



Other problems in solar physics have been 

 solved by the same method. The existence of 

 sun-spots has long been known ; they were, 

 indeed, familiar to the Chinese in very early 

 times, and, in the middle of the nineteenth 

 century, their periodic increase and decrease in 

 a cycle of ten or eleven years was noted by 

 Western observers, and a coincident period of 

 terrestrial magnetic phenomena was established. 

 The structure and properties of sun-spots were 

 then seen to possess more than a local solar 



