2-4] Survey of the Problem 5 



(1) Irregular nebulae, such as the great nebula in Orion (N.G.C. 1976). 



(2) Planetary nebulae, a class of nebulae of apparently spheroidal or 

 ellipsoidal shape, many shewing detailed features and formations in addition. 

 They are few in number, less than 150 having been discovered out of 15000 

 nebulae so far investigated*. As a rule they shew bright-line spectra, sug- 

 gesting that they are masses of hot gas, shining by their own light. Some 

 typical examples of Planetary nebulae will be found illustrated on Plate I. 



(3) Ring nebulae, such as the well-known nebula in Lyra (N.G.C. 6720). 

 Many astronomers believe that these are not true rings but ellipsoidal shells 

 seen in projection; the reason for this view is mainly that these formations 

 are never seen edgewise or nearly edgewise (see Plate I). 



(4) Elliptical, elongated, lenticular and spindle nebulae. These are 

 terms commonly employed to describe the observed shape of nebular masses. 

 A number of nebulae originally classified as spindle-shaped are probably 

 merely spirals seen edgewise, as has been suggested by Slipherf and others. 

 Descriptions, with excellent photographs of these and other types of nebulae 

 will be found in a recent paper by F. G. Pease J (see also Plate III). 



4. Beyond the information obtainable from their appearance and spectra, 

 we have but little knowledge as to the nature, motions or constitutions of 

 these various nebular systems . Many of the spirals have velocities in space 

 which are enormously greater than any Bother class of velocities of which we 

 have any experience, a circumstance which gives some support to the view 

 that they may be regarded as "island universes," each comparable in scale to 

 the universe of stars of which our sun is a member. 



Thus for the Andromeda nebula there is consistent evidence of a velocity of 

 approach of about 300 kms a second, Slipher|| determining this velocity as 300 

 kms a second, Wright 1 as 304 kms a second and Pease** as 329 kms a second. 

 Many spirals have still greater velocities; thus Pease attributes a velocity 

 of recession of about 1180 kms a second to the nebula in Virgo (N.G.C. 4594) f-f- 

 while Slipher finds a velocity of recession of 1120 kms a second for the nebula 

 in Cetus (N.G.C. 1068)++. The general average velocity is between 300 and 

 400 kms a second say twenty times the general average velocity of a star in 

 our universe. Regarding these nebulae as "island universes," it ought of 

 course to be possible to determine the motion of our own galactic system in 



:: W. W. Campbell, Science (1917), p. 521. f Lick Obs. Bull. No. 62. 



J Astrophys. Journ. 46 (1917), p. 24. See also W. W. Campbell, Science, 45 (1917), 

 pp. 513548. 



A short summary will be found in the E.A.S. Monthly Notices,'!! (1917), p. 375. 



|| Lowell Obs. Bull. No. 58 (1913). IT Popular Ast. 23 (1915), 36. 



** Journal Roijal Ast. Soc. Canada, Sept. 1915. 

 ft Astrophys. Journ. 46 (1917), p. 41. 

 JI Lowell Obs. Bull. 80 (1918). 



