14 



Proceedings of the Roi/al Irish Avademy. 



+ 30°-243 Mag. 9-3 : 



Leicleu, V' 29"' 24^-68 (4) + 30^ 15' 21"-7 (4) 



Cambridge 

 Kiistner, 



24-47 (2) 

 24 -78 (2) 



18 -6 (2) 

 2] -1 (2) 



Adopted, 1 29 24 '73 + 30 15 21-40 



+ 30°-245 Mag. 8-6 : 



Leiden, 1'^ 29'" 44^-06 (2) + 30° 15' 30"-3 (2) 

 Cambridge, 44-11 (2) 29 -4 (2) 



Bonn VI., 44-10 (2) 31 -0 (2) 



+ 5^-03 + 33"-01. 



Adopted, 1 29 44 -09 + 30 15 30 -23 

 + 30°-247 Mag. 9-4 : 



Dunsink, 1'^ 30'« 9^-77 (4) + 30° 13' 55"-10(4) 



+ 5^-03+ 32"-98. 

 + 5^-04 + 32"-94. 



These places are represented as follows (Mer.-Phot.) : — 



+ 30°-238 

 241 

 243 

 245 

 247 



-0^-12 - 0"-67 

 -0-02 + ] -10 

 -0-14 +0 -50 

 - 0-12 +0 -60 

 -0-08 +0 -63 



In order to I'orm some idea of the accuracy of the directly 

 measured coordinates x and y, I have computed the probable error of 

 one determination of either coordinate from the differences between 

 the single results, taking first the above-mentioned nine stars, next 

 100 stars described as 2^B or pi^, and finally one hundred F or vF 

 stars. These were chosen without any regard to the amount of 

 difierence between the results, but all objects which were described 

 as nebulous "patches" or "knots" were excluded. The probable 

 errors Avere found to be : — 



For stars 8 •0-9-5 mag., ± 0". 37 



For stars or stellar condeusations, 11-12 mag. ± ± 0-35 



For ditto. 13-14 mag. ± ± 0-39 



But obviously this does not properly represent the degree of 

 accuracy of the work as far as the bright stars are concerned, since it 

 is a priori to be expected that the bisection of the very large discs, 

 produced by an exposure of the plate during an hour and a half, must 

 be affected by systematic errors, particularly as some of the discs are 

 not perfectly circular. For instance, on the plate measured, while 



