ASTRONOMY 



531 



spirals at distances of i,ooo and 10,000 light-years^respectively 

 would have annual proper motions of o'''48 and o'''048 respec- 

 tively. The former value is certainly too great to be admissible ; 

 the latter could scarcely be detected at present. It is therefore 

 only possible to conclude from this argument that the distances 

 are probably equal to, or greater than, 10,000 light-years. A 

 second line of evidence, and perhaps the most promising at 

 present available, depends upon a comparison of the linear 

 velocities of rotation of spirals seen edgewise with the angular 

 velocities of rotation found by van Maauen in the case of a 

 few spirals observed broadside on. If the two rotations are 

 assumed comparable, the distances deduced are of the order 

 of 25,000 light-years. But this evidence is at present very 

 slender, and it is doubtful to what extent the results can be 

 regarded as typical of the large numbers of spirals in general. 



Another argument has been based upon a comparison of 

 the magnitudes at maximum of the galactic novae and of novae 

 observed in the Andromeda nebula. Curtis gives the following 

 figures : 



Curtis argues that these figures support a distance of 500,000 

 light-years rather than of 20,000 light-years. But it should 

 be emphasised that the distances of the four galactic novae have 

 only been determined with a relatively large uncertainty ; and, 

 further, these four cannot be accepted as generally typical of 

 galactic novse. Novae which at maximum only attain a low 

 apparent magnitude may easily be missed, whilst probably all 

 novae which have recently appeared in the Andromeda nebula 

 have been detected. The result would be to increase the 

 discrepancy in the two sets of figures. Whilst, therefore, it 

 appears probable that the distance of the Andromeda nebula 

 exceeds 20,000 light-years, it appears very doubtful whether 

 the distance can approach 500,000 light-years. But if not, the 

 nebula would come within the limits of the Galactic System 



