174 Prof. C. Pritchard. On the Capacities of two [May 3, 



from the centre of the plate, and in order to ascertain this, I made a 

 preliminary examination of the optical quality of the field, by the 

 method which I have described in vol. 47 of the ' Memoirs of the 

 Royal Astronomical Society' (p. 238). This method consists in 

 shifting the images of the same pair of stars to widely different 

 localities in the field of view, and it was argued that so long as the 

 measured angular distances between these pairs remained sensibly 

 the same, i.e., within the known and unavoidable limits of observa- 

 tional error, so long might the optical field of view be relied upon as 

 sensibly accurate. 



Fig. 1. 



ABCD represents the photographic plate where AB is 4 inches, 

 and subtends an angle of 1° 55' at the centre of the x ^-inch mirror. 

 A pair of stars of approximately the seventh magnitude was selected, 

 and photographed near the centre of the plate, as at (a), with an 

 exposure of five minutes. The telescope was then moved approxi- 

 mately fifteen minutes to the south, and a second photograph taken, 

 by which this same pair was removed to (b). This process was 

 repeated again and again in northerly, easterly, and westerly direc- 

 tions, till after thirteen exposures this same pair of stars was 

 dotted about the plate as in the diagram. This same process was 

 repeated on three plates on the same night (March 3, 1888). The 

 distances between each pair were then measured, and the means of 

 five measures of each pair were taken as the adopted measures for each 

 pair respectively. The results are as follows : — 



