212 A CENTURY'S PROGRESS IN ASTRONOMY. 



This has given an enormous extension to our 

 knowledge of the nebulae. But even this is not 

 all. Keeler found on his plates numerous points 

 of light which seem to be also nebulae, either too 

 small or too remote to appear as such. Appar- 

 ently, however, they are not stars. Keeler's 

 work convinced him that, on a modest estimate, 

 there must be at least one hundred and twenty 

 thousand new nebulae within reach of the 

 Crossley reflector. Half of these, he announced, 

 were probably spiral. An idea of the vast 

 importance of Keeler's work may be gained if 

 we reflect that the observations of all the earlier 

 astronomers resulted in the discovery of six 

 thousand nebulae. The investigations of Keeler, 

 in all probability, were the means of adding 

 120,000 more. 



Many observations have been made on nebulae, 

 for the purpose of ascertaining their proper 

 motions but without success. Measurements 

 were made by D' Arrest in 1857 and by Burn- 

 ham in 1891, but none of these revealed any 

 motion of the nebulae across the line of sight. 

 Even the new spectroscopic method of deter- 

 mining motions in the line of sight, in the hands 

 of Huggins, failed in the case of the nebulae. 

 With the great Lick refractor at his disposal, 

 Keeler attacked the subject in 1890, and 



