;8 



FIG. 44. Spiral Nebula M 33 TriaiiRuli. 



on an incomparably greater scale than that of our 

 own solar system, which would shrink to the di- 

 mensions of a pin-point if seen at the same dis- 

 tance. This distance is so great that in spite ot the 

 realistic appearance of motion in these spirals, no 

 evidence of change in form has yet been detected. 

 But Slipher has found displacements of their 

 spectral lines indicating rotational motion, and 



