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



Distances between the Pair of Stars, corrected for Refraction. 



~P/a a W inn 1 



of star . . J 



a. 



b. 



c. 



d. 



e. 





9- 



Plate I. . 

 II.. 

 III.. 



152-00 

 151 -88 

 152-08 



15l"92 

 2-07 

 2-17 



151-93 



1- 81 



2- 05 



15V99 

 152-10 

 2-03 



152-07 

 1'88 

 1*96 



152 22 



1- 95 



2- 07 



152-02 

 2-02 

 2-44 



Mean. . . 



151 -99 



152 -05 



151-93 



152-04 



151 -97 



152 -08 



152 -16 



"PriQif inn 1 



of star . . j 



a. 



V. 



c\ 



d'. 



e r . 



/'■ 





Plate I. . 

 II.. 

 III.. 



]52 / -00 

 151-88 

 152 -08 



152"l4 

 2-19 

 2-07 



152 // 05 

 2-08 

 2-00 



2-36 

 2-24 



15l'-89 



1- 83 



2- 05 



151-91 

 1-96 

 1-76 



152 / -05 



1- 86 



2- 10 



Mean. . . 



151 -99 



152-13 



152-04 



152-26 



151-92 



151 -88 



152-00 



The following table exhibits the deviations of the intervals from 

 the central interval at different positions on the plate : — 



Due North 15 -0 V 02 



30.. +0-09 



50 +0-17 



Dae South 15 +0*06 



30 -0-06 



48 +0-05 



Due East 15 -0-07 



30 -0-11 



46 +0-01 



Due West 15 + 0'14 



30 +005 



52 +0-27 



When it is remembered that the unavoidable error of such measures 

 is about 0"*2 (where 0*0001 in. is equivalent to / ' , 17), the only conclu- 

 sion to be drawn is that to the extent of the field impressed on this 

 plate of 1° 55' square, there is no perceptible or measurable distortion 

 in the apparent distance of these pairs, and in fact that small 

 measured distances may be relied upon throughout the field ; and 

 thus, if a few stars are scattered about the plate with known co- 

 ordinates, those of all the rest may be conveniently determined with 

 great accuracy. 



