386 Rutherfurd Photographic Ileaswes. 



tion is the result of a,t least twenty measures of distance and 

 twelve of position angle for each plate recorded in this column 

 two. With regard to the probable errors of these resulting posi- 

 tions the same remarks appl_y to these plates as to those of ^ 

 Gygni."^ 



The precessions and secular variations depend on the constants 

 of Struve, and were computed with the aid of Folie's tablesf 

 with the necessary differential corrections applied to make them 

 good for 1872. 



With regaixl to Scheiner's remarks^ on the correction for the 

 refraction formulffi suitable for visual observations to make them 

 applicable to photographic work, I would sa}' that I have not ap- 

 plied such corrections in the present paper, for the reason that the 

 Rutherfurd plates, being made more than twenty years ago and 

 previous to the use of '• dry plates," would probably not furnish 

 the same coefficient of refraction for the violet rays as would the 

 plates that have been used in Scheiner's investigation. In fact, 

 this surmise is in part confirmed by a difference of over two per 

 cent, in the coefficient as determined from Rambaut's plates and 

 by Scheiner.]; In the present paper, however, an inspection of 

 Table Y., column four, will show a more practical justification for 

 ignoring a correction which would, with our present knowledge of 

 the plates used by Rutherfurd, be of uncertain value, namely, in the 

 fact that even in the case of the star giving the largest mean re- 

 fraction for all the plates, a correction of ^-^ of that mean refrac- 

 tion would in the most extreme case make a change of less than 

 .^004 in «, and less than ."01 in 0, owing principally to the fact 

 that the plates were exposed at small zenith distances as seen in 

 Table I. 



*Anu. N. Y. Acad. Set., Vol. YI., p. 334, ef. seq. 



tDouze Tables pour le Calcul des Reductions Stellaires, Mem. Soc. Roy. des 

 Sci. de Liege, t. x. 1883 supplement. 



:Ast. Xachr. Bd. 136, N. 3255, p. 248. 



+ * 



