326 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



Plate U. 



Comparison with Oxford Plate K 2 . 



Since Plate U shows some good images it has been examined, although owing to the 

 absence of star 8 the weight is small. The measures were made at Principe. 

 Theoretical plate-constants 



a = 



l,d=+8-Q, e=-f37-7. 



In this case it is not possible to determine the orientation with sufficient accuracy from 

 the ic-measures ; the value here applied is an arbitrary preliminary value. We 

 accordingly make a least-squares solution from both x- and ^/-residuals to determine 

 the correction to the orientation, 80, as well as Sc, 8/ and SK. 

 The result is 



80 =+2, 8*c=+121. 

 This gives the deflection 



2" -90. 



The probable error is, however, 0"-87, so that the result is practically worthless. 

 Further, it is much more likely to be affected by systematic error than the previous 

 results. 



The large probable error is partly due to the large residuals which are greater than 

 in the previous measures ; in particular star 3 is unduly faint. If the same accuracy 

 had been obtained, the theoretical weight would have been half that of plates W and X : 



