272 IN STARRY REALMS. 



happen to know the distance of the star from the earth, 

 and with this important element we are, in the majority of 

 cases, quite unacquainted. 



Quite otherwise is it in the spectroscopic method of 

 measuring proper motions along the line of vision ; what 

 we now obtain is not a measurement of arc on the sky, it 

 is the velocity in miles per second that is given to us, and 

 this, be it observed, not after the lapse of a long period of 

 time between the first observation and the last, but directly, 

 so that by turning the spectroscope on one star we may, 

 for example, say that it is approaching at the rate of ten 

 miles a second, while the same method applied to another 

 star may show a recession at the rate of ten, fifteen, or any 

 other number of miles per second. To obtain a complete 

 knowledge of the movement of the stars we require both 

 the spectroscopic method and the old-fashioned telescopic 

 method. By the union of the two classes of measurement 

 a complete knowledge is obtained of the way in which each 

 star is moving. 



Let me now elucidate with sufficient detail the principle 

 of the very beautiful process on which the spectroscopic 

 method of determining movements on the line of sight is 

 based. Light is known to be caused by waves in that 

 mysterious fluid, the ether. The ether fills all space, at 

 least, so far as space contains objects visible to us, and the 

 light from a star is caused by ethereal waves which have 

 trembled along an appalling distance of many billions of 

 miles from their origin in the glowing star to the retina, 

 where they set the nerves in vibration. 



Everyone who has stood on a sea-beach and watched the 

 sea rolling in can find no difficulty in comprehending 



