M.-P.— Vol. I.] CRAWFORD— CONSTANT OF REFRACTION. 193 



Addendum. 



The table on page 189 shows a large range in the values 

 of A, viz., from -j-o. 00662 to — 0.00584. Upon plotting 

 these values, using the zenith distance z for abscissa, and 

 A for ordinate, it is easily seen that A varies quite uniform- 

 ally with the zenith distance. A straight line, inclined 

 about 145 to the zenith distance axis, and cutting it at z = 

 about 55 , appears to represent A very well. Therefore, 

 assuming Z to be the zenith distance for A = o, we can set 

 up an observation equation of the following type for every 

 star: 



loga=loga -|- [ Z — z ] x > 



or 



where 



loga — loga =A=Zx — zx — D — zx, 

 D=Zx, 



and where a is the a of the tables used (Pulkowa). 



Equations of this kind were, accordingly, formed and 

 solved for Z and x by the method of Least Squares. 



