ASTRO-PHYSICS 271 



is but the ten-thousandth part of the velocity 

 of light. This velocity of approach, then, 

 would involve a change of the ten-thousandth 

 part in the period of vibration of the incident 

 light. The whole visible spectrum, from the 

 red to the violet of the rainbow, includes a range 

 of frequencies of about an octave, that is, the 

 period of vibration of the extreme red is about 

 double that of the extreme violet. A velocity 

 equal to that of the earth, then, would involve 

 a change in position of a spectral line of about 

 the five-thousandth part of the total length of 

 the spectrum. Many stars are approaching or 

 receding from the earth at velocities higher than 

 that which we have taken as an example, but 

 still the changes in position to be measured 

 are very small, and refined methods and great 

 experimental skill are needed for accurate results. 



The problem of determining the movement 

 of a star travelling along the straight line joining 

 it to the observer would, before this principle 

 was discovered, have seemed one of the most 

 hopeless problems which a cynical scientific 

 sceptic could propose for solution to the 

 physicist. Yet such problems are now solved 

 daily, or rather nightly ; solved, indeed, much 

 more readily than they could be if the star were 

 moving across the line of sight. In the latter 

 case, even if a knowledge of the distance makes 

 the determination possible, prolonged observa- 

 tions are needed, extending over months or 

 years, till the movement becomes apparent at 

 the distance of the earth. Many stars are so 

 distant that no such cross movement could be 



