620 Proceedings of the Royal Irish Academi/. 



a number of trustworthy results respecting the personal errors of faint 

 stars. 



Closely connected with the question of personality in observations 

 of faint stars is another important matter, to which the great number 

 of nebular observations, made during the last twenty-five years, has 

 turned our attention. "We alhide to the constant differences between 

 the right ascensions found by different astronomers. It is especially 

 Schonfeld, whose results, published in Vol. I. of the "Mannheim Ob- 

 servations," show a remarkable deviation from those of all other 

 observers of nebulae, as the following equations show : — 



Schonfeld - Laugier = - 0^"21. 



- d' Arrest = - 0-38.^ 



- Schmidt = - 0-39. 



- Schultz =-0-30.t 



- Oppolzer =- 0-38.1 

 -Yogcl =-0-21. 



Although it seemed unquestionable that Schonfeld's right ascensions 

 are too small, I thought it would be of interest to compare them with the 

 " Micrometrical Observations of 500 nebulae by Dr. Herman Schultz," 

 which were published in Upsala, in 1874, in order to see, whether the 

 very striking difference between the Mannheim and the few earlier 

 Upsala observations would appear again, when all the observations 

 of later years were employed in the comparison. It is safest only to 

 compare objects which have been determined by both observers by 

 means of the same comparison-star, as the small differences between 

 the positions of the different star catalogues might produce here too 

 great an effect upon the value of the small personal equation. Of 

 the 163 objects which occur both in Schultz's and Schonfeld's obser- 

 vations, I, therefore, only took 114, which have been compared with 

 the same star. Of these 114 the neb. h 393 could not be used, as it 

 seemed most probable that a different point had been observed. The 

 remaining 113 equal differences nebula-star gave for the equation 

 •Schonfeld - Schultz 



Aa cos S = - 0^-338. 



As only thirteen nebulae gave a very small positive value Aa cos, 

 there is no doubt, that one of the two observers has a different way of 

 estimating the transits of stars and nebulae. 



That these indications of personal equations in nebular observations 



* Mean of all the comparisons contained in the " Observationes Havnienses." 

 This number is, of course, of less value than the others, as d' Arrest only observed 

 most nebulae approximatively. The comparison bet^reen Schonfeld and d' Arrest's 

 " First Series " (Leipzig, 1856) gave a similar difference, which, however, disap- 

 peared when only nebulae observed with the same star were compared. 



t Astron. Nachr., Ixiii., No. 1504. 



