THE ORIGIN OF THE EARTH. 49 



vastly transcend those of the solar system, and they cannot be taken 

 as precise examples of the solar evolution. Because of these vast 

 dimensions and of the probable feebleness of control of the central mass, 

 which often appears to be itself quite tenuous, a rapid motion cannot 



Fig. 20. — The great nebula in Andromeda, seen, apparently, obliquely; the greatest 

 of the spiral nebulae. The nucleus is highly preponderant and spheroidal; the 

 knots are less pronounced than in the preceding, and nebulous haze is more abun- 

 dant relatively. The spectrum is continuous, with some dark lines, implying that 

 the central nucleus is akin to the sun. No parallax has been determined, but the 

 distance is inferred to be great and the dimensions immense, as many small stars 

 appear to be this side the nebula. This nebula has sometimes been suspected to 

 be in reality a stellar system outside our own. Two smaller spheroidal nebulse 

 are shown which may or may not be connected genetically with the great one. 

 (Photo, by Ritchey, Yerkes Observatory.) 



well be assigned to the arms. Seen from the immense distances at 

 which the nebula? seem to be placed — no parallax having been as yet 

 detected — changes of position must necessarily be slow in revealing 

 themselves to observation. It is to be hoped, however, that the present 

 rapid progress in the perfection of instruments and of skill will soon 



