1888.] Silver on Glass Mirrors of different Focal Lengths. Ill 



Distances of Atlas from Pleione, corrected for Refraction. 



Position of star. . . . 



a. 



b. 



c. 



d. 



e. 



II 



Ill 



301 '-21 

 115 

 1-40 



301 / -47 

 1-53 

 1-32 



30l"52 

 1-74 

 1-60 



30l'-29 

 1-53 

 1*47 



30l"63 

 1-80 

 1-52 



301 -25 



301 -44 



301 -62 



301-43 



301 -65. 



Position of star. . . . 





9- 



h. 



i. 





Plate I 



II 



301-37 

 1-41 

 1-29 



301 / -32 

 1-60 

 1-39 



30l"58 

 1-79 

 1-71 



302"- 17 

 2-01 

 1-92 



301-36 



301-44 



301-69 



302 -03 



The following table exhibits the deviations of the intervals from 

 the central interval at different positions on the plate : — 



Due North 42 +CM4 



K West 51 +078 



Due West 32 +0 37 



S. West 44 +0-11 



Due South 37 +019 



S. East 48 +0-40 



Due East 39 +0*18 



N. East 45 +019 



It should be observed here, that while the linear discrepancies of 

 measured distances are the same as those with the y^-inch mirror, 

 they indicate larger angular discrepancies in the ratio of 3 : 2. 

 Nevertheless, the examination of these angular discrepancies exhibits 

 evident traces of distortion, sufficient to render extreme accuracy of 

 measures unattainable without the great difficulty of an extensive 

 tabulation ; in other words, the comparative short focus of this 

 mirror is not well adapted to the purposes of accurate measurement. 

 Perhaps I ought here to refer to the very careful examination of the 

 field of the Grubb refractor of 12^ inches aperture and 176 inches 

 focal length, recorded in the 'Memoirs of the Royal Astronomical 

 Society,' vol. 47, p. 238, in which it appears that no absolute reliance 

 could be assigned to measures extending beyond 12 minutes from the 



