( 695 ) 



These p.m. were applied by Mönnichmeyer before he derived his 

 definilive differences in a and rf (Neb.-Star). In no other case a 

 correction for the p. m. of the comparison stars was applied. 



The majority of the observers used the ringmicrometer. 



The principal error to be feared for observation with this micro- 

 meter is the personal error in right ascension. Mönnichmeyer has 

 devoted the utmost care to their determination. Notwithstanding this 

 it may be considered a fortunate circumstance that this error has no 

 influence on (he result for the mean parallactic motion, at least in 

 the ideal case that the nebulae are distributed uniformly over the 

 right ascensions from to 24 hours. 



For it seems highly probable that the distance of the nebulae is 

 not systematically different in the different hours of right ascension. 

 This being so the personal error will vitiate the parallactic p.m. of 

 the nebulae at the same distance in right ascension on both sides 

 of the apex, to the same extent but in opposite directions. 



It is true that the distribution in right ascension is far from being 

 uniform ; still we may be sure that whatever residual personal 

 errors may still exist in the materials of Mönnichmeyer, must appear 

 considerably diminished in the result. Meanwhile I have tried to 

 obtain some idea about the possible amount of these residual errors 

 in the following way. 



I computed the average proper motion in right ascension for each 

 hour separately. Taking the simple mean of all these hourly averages 

 we may expect to get a result in which not onl}' the peculiar proper 

 motions, but, as explained just now, also the parallactic motions 

 shall have vanished. 



