DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 319 



Star 7 is much the brightest. Stars 1, 6, 11, 13 are rather bright. Stars 2, 4, 10, 12 

 are fainter and more comfortable to measure. Stars 5 and 8 are very faint. Arcturus 

 is on the plates but is much too bright to measure. No measures have been rejected. 



The determination of the deflection on the eclipse plates is based on the declinations 

 (y), and the last column of Table XII. shows that on the check plates the ^-comparisons 

 are free from any serious systematic error. 



Star 7 is of particular interest ; its position near the centre of the field corresponds to 

 that of KI, K- 2 Tauri in the eclipse field, from which the greatest deflection is expected. 

 The images (which are not quite round) have the same characteristic shape. Further, 

 the brightness of No. 7 corresponds with, but exaggerates, the brightness of K I Tauri 

 which is the brightest star in the eclipse field. It is therefore a valuable check to find 

 that its systematic error in declination is insignificant compared with the displacement 

 (of the order of 1") afterwards found for K-J and ic a Tauri. 



The systematic errors in right ascension are larger (probably through imperfect driving 

 of the clock). They may affect the displacement indirectly through the orientation 

 constant, but with much reduced effect. Allowing for this reduction in importance there 

 appears to be nothing to trouble about. 



The primary purpose of the check plates is thus fulfilled. They show that photographs 

 of a check field of stars taken at Oxford and Principe show none of the displacements 

 which are exhibited by the photographs of the eclipse field taken under precisely 

 similar instrumental conditions. The inference is that the displacements in the latter 

 case can only be attributed to presence of the eclipsed sun in the field. 



33. We turn now to the differences of scale between Oxford and Principe, which are 

 given by the plate-constants a, b, d, e determined from the measures. As determined, 

 these include the effects of differential refraction and aberration. The latter corrections 

 were calculated for each plate by the usual formulae and applied, so as to determine 

 the corrected plate-constants, a' , b' , d' , e free from differential refraction and aberration. 

 Due allowance was made for the change in the coefficient of refraction owing to the 

 difference of barometer and temperature (about 40) between Oxford and Principe. 

 The results are as follows (in units of the fifth place of decimals) : 



TABLE XIII. Check Plates, Plate-Constants. 



