The Milky Way 



Keeler has been able to determine the radial 

 velocity of about fifteen nebulae by observing 

 the change of position of the spectral lines; 

 these velocities are about 50 to 60 kilometres 

 per second. If, as would appear probable, the 

 tangential velocities are of the same order of 

 magnitude, it may perhaps some day be possible 

 to compute the distance which separates us 

 from these nebulae. If this distance does not 

 exceed twenty million times the distance from 

 the earth to the sun, the tangential motion 

 will have, in a hundred years, transported 

 these nebulae a distance in the heavens equal 

 to that covered by one of the spider-threads 

 stretched across the focus of our telescopes; 

 probably a century would be insufficient to 

 ascertain this motion, but astronomers are 

 patient, and we may also hope that they will 

 succeed in discovering new methods and in 

 improving the old ones. 



When Herschel commenced to attack the 

 nebulae with instruments of increasing power 

 he observed that some of them were reduced 

 to agglomerations of stars to which he gave 

 the designation of resolvable nebulce, and it was 



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