DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 317 



The Check Plates. 



31. In addition to the eclipse field, a check field was photographed both at Oxford 

 and at Principe. The field chosen included Arcturus, so that it was easily found with 

 the coelostat. Its declination was nearly the same as that of the eclipse field, and it 

 was photographed at the same altitude at Principe in order that any systematic error, 

 due to imperfections of the coelostat mirror or other causes, might affect both sets of 

 plates equally. The primary purpose was thus to check the possibility of systematic 

 error arising from the different conditions of observation at Oxford and Principe, and 



, from possible changes in the object-glass during transit. Unlike the Sobral expedition, 

 we were not able to take comparison photographs of the eclipse field at Principe, 

 because for us the eclipse occurred in the afternoon, and it would be many months 

 before the field could be photographed in the same position in the sky before dawn. 

 The check plates were therefore specially important for us. 



As events turned out the check plates were important for another purpose, viz., to 

 determine the difference of scale at Oxford and Principe. As shown in the report of the 

 Sobral expedition, it is not necessary to know the scale of the eclipse photographs, since 

 the reductions can be arranged so as to eliminate the unknown scale. If, however, a 

 trustworthy scale is known and used in the reductions, the equations for the deflection 

 have considerably greater weight, and the result depends on the measurement of a 

 larger displacement. On surveying the meagre material which the clouds permitted us 

 to obtain, it was evident that we must adopt the latter course ; and accordingly the 

 first step was to obtain from the check plates a determination of the scale of the Principe 

 photographs. 



32. All the measures were made by Prof. EDDINGTON with the Cambridge measuring 

 machine.* An Oxford and a Principe plate were placed film to film so that the images 

 of corresponding stars nearly coincided this was possible because the Oxford plates 

 were taken direct, and the Principe plates by reflection in the ccelostat mirror. 



The small differences Ax and Ay, in the sense Principe Oxford, were then measured 

 for each star. Eight settings were made on each image ; for half of them the field was 

 rotated through 180 degrees by the reversion prism. Five pairs of plates were 

 measured, and the measures are given in Table XI. 



' Monthly Notices, R.A.S.,' vol. LXI, p. 444. 



2x2 



