218 PROGRESS OF SCIENCE IN THE CENTURY. 



But while the use of the spectroscope revealed the 

 presence of certain chemical elements in the stars, 

 and distinguished gaseous from star-cluster nebulae, 

 it led to an even more important achievement the 

 detection and measurement of the motion of certain 

 stars in the line of sight. We cannot briefly explain 

 the suggestion of Christian Doppler (1848) that 

 " the colour of an object should be affected by the 

 motion of the source, becoming more violet as the 

 object approached, and inclining toward red as it 

 receded from, the observer," * or the method of 

 Fizeau (1848) by which the displacement of the dark 

 lines in the spectrum was used as an index of 

 approach or recession. These led to the work of Sir 

 William Huggins who announced in 1868 that he 

 had found spectroscopic evidence (a minute displace- 

 ment of a dark hydrogen line) of the recession of 

 Sirius and estimated the rate of this recession (from 

 the sun) at 29^ miles per second. He extended the 

 discovery to thirty other stars and confirmed the 

 method by the spectroscopic study of Venus at 

 different times when the planet was known to be 

 moving towards or away from the earth. 



It is interesting to notice that displacement of 

 lines has also been detected in the observation of sun- 

 spots, and has led to the conclusion that these are 

 due to downrushes of gases. 



From 1870 onwards, the splendid work of Huggins 

 was continued by Hermann Vogel, at Potsdam, who 

 in 1887 availed himself of the valuable aid afforded 

 by the dry gelatine plate and the microscopic ex- 

 amination of its photographic record of the spectrum. 

 The motions of approach and recession of many 

 stars were thus calculated with great accuracy, and 

 * Fison, Recent Advances, 1898, p. 200. 



