Mat 25, 1917] 



SCIENCE 



537 



Fig. 26». Planetary N^jjuia N.G.C. 7009. 

 Composite drawing by Curtis from photographs 

 made with the Crossley Eeflec.tor of the Lick 

 Observatory. With the slit of a 3-prism spec- 

 trograph placed on the longer axis, the bright 

 nebular lines were found to be inclined to the 

 direction of the slit, owing to the rotation of the 

 nebula, as shown (exaggerated) in the upper 

 part of this figure, and as reproduced from the 

 spectrogram in Pig. 26b. 



masses, in accordance with the simple laws 

 of pliysics, and the planets and their satel- 



lites of our system, as they exist to-day, are 

 the result. 



I see no reason to question that a spiral 

 nebula could originate in this manner : the 

 close passage of two massive stars could, in 

 my opinion, produce an effect resembling 

 a spiral nebula, quite in accordance with 

 Moulton's test calculations on the subject. 

 Some of the spirals have jjossibly been 

 formed in this way ; but that the tens of 

 thousands of sj^irals have actually been pro- 

 duced in this manner is another question, 

 and one which, in my opinion, is open to 

 grave doubt. The distribution of the spir- 

 als seems to me to negative the idea. If the 

 close approaches of pairs of stars are pi*o- 

 ducing the spirals we should expect the 

 .spirals to occur and to exist ijreeminently 

 in and near the Milky Way structure, for 

 that is where the stars are ; and that is pre- 

 cisely where we do not find the spirals. 



I think it is more probable that our 

 stellar system as a whole is a spiral nebula, 

 or has analogies to a spiral nebula, and that 

 our solar system has been formed from an 

 insignificant detail of spiral structure, than 

 that our sun and its sj^stem of planets and 

 moons should be the evolved product of an 

 entire spiral nebula. Of course we have not 



Fig. 26/;. The First (right) and Second (left) Green Lines of 

 Nebulium in the Spectrum ' of N.G.C. 7009. 



[Enlargement approximately 20-fold. The six lines at the 

 upper and lower edges of the figure are the reference spectrum of 

 titanium. 1 



