364 Proceedings of the Royal Irish Academy. 



of Gr. 1618. It will at the outset be of importance to show that the 

 comparison star has not a proper motion comparable in magnitude 

 with that of Gr. 1618. 



This will appear most clearly by a comparison of the recent obser- 

 vations with those made by Argelander on March 21st, 1843, as reduced 

 in Argelander-Oeltzen to the epoch 1842*0. From an approximate 

 reduction of my observations I conclude that at the epoch 1878'0 the 

 distance of + 50°, 1724 from Gr. 1618 is 198"-1, and the position angle 

 is 201° 42'-8, From this I find 



Aa = 7^-603, 



AS = 184"-1 ; 



but from Argelander-Oeltzen, we have 



Aa = 12^-4, 



AS = 203"-9. 



"We apply to these differences the annual proper motion of Gr. 1618, 

 viz. - 0^-1 390 in E. A. and - 0"-501 in Decl. During the thirty-five 

 years that have elapsed since Argelander' s observations the corrections 

 will amount to - 4^*87 and - 17"'5, respectively. We thus find for 

 the values in 1878 



Aa = 7'-5, 



AS = 186"-4. 



It is therefore evident that + 50°, 1724 cannot have a proper mo- 

 tion which even in thirty-five years amounts to much more than 0''103 

 R. A., or 2""3 in Declination. Compared with the large proper motion 

 of Gr. 1618 these quantities are inappreciable. We may, therefore, 

 conclude as a first approximation that the absolute proper motion 

 assigned by Argelander to Gr. 1618 coincides with its proper motion 

 relatively to the comparison star + 50°, 1724. 



The annual decrement of the distance from Gr. 1618 to + 50°, 1724, 

 in consequence of the proper motion of the former, is 0"-9579. The 

 position angle decreases for the same reason at the rate of 18''398 

 annually. 



In the first series of observations now to be discussed, measurements 

 were made on fifty-five nights both of distance and position. In the 

 present Paper the measurements of the distance alone are discussed. 



The following Table contains the observations of the distance, cor- 

 rected in the usual manner, for the effects of refraction and tempera- 

 ture of the micrometer screw, and reduced to the epoch 1878*0. The 

 corrections for proper motion have, however, not yet been applied. 



