474 



Royal Society 



be, say, 10 miles per second from, the nebula, then the nebula 

 would not be receding with a velocity greater than 15 miles per 

 second ; but the nebula might be approaching with velocity as great 

 as 35 miles per second, because 10 miles of this velocity would 

 be destroyed by the earth's motion in the contrary direction. 



The observations seem to show that the gaseous nebulae as a 

 class have not proper motions so great as the bright stars. It 

 may be remarked that two other kinds of motion may exist in the 

 nebulae, and, if sufficiently rapid, may be detected by the spectro- 

 scope : — 1. A motion of rotation in the planetary nebulae, which 

 might be discovered by placing the slit of the instrument on oppo- 

 site limbs of the nebulae. 2. A motion of translation in the visual 

 direction of some portions of the nebulous matter within the 

 nebula, which might be found by comparing the different parts of 

 a large and bright nebula. 



Sir "William Herschel states that " nebulae were generally de- 

 tected in certain directions rather than in others, that the spaces 

 preceding them were generally quite deprived of stars, that the 

 nebulae appeared some time after among stars of a certain con- 

 siderable size and but seldom among very small stars, that when 

 I came to one nebula I found several more in the same neigh- 

 bourhood, and afterwards a considerable time passed before I came 

 to another parcel"*. 



Since the existence of real nebulae has been established by the 

 use of the spectroscope, Mr. Proctorf and Professor D'ArrestJ 

 have called attention to the relation of position which the gaseous 

 nebulae hold to the Milky Way and the sidereal system. 



It was with the hope of adding to our information on this point 

 that these observations of the motions of the nebulae were under- 

 taken. 



In the following list the numbers are taken from Sir J. Herschel's 

 ' General Catalogue of Nebulae.' The earth's motion given is the 

 mean of the motions of the different days of observation. 



No. 



h. 



H. 



Others. 



Earth's motion from Nebula. 



1179 



360 





M. 42 



7 miles per second. 



4234 



1970 





2.5 



12 „ 



4373 





IV. 37. 





1 » 



4390 



2000 



, , 



2.6 



2 „ 



4447 



2023 





M. 57 



3 ,; 



4510 



2047 



IV. 51. 



, , 



14 „ 



4984 



2241 



IV. 18. 



•• 



13 „ 



* Philosophical Transactions, 1784, p. 448. 



f Other Worlds than Ours, pp. 280-290. 



j Astronomische Nachrichten, No. 1908, p. 190. 



