DETERMINATION OP DEFLECTION OF LICITF IJYTHE .SUN’S CRAVITATfONAT. FIELD. 
17 
The Check Plates. 
31. In addition to tlie eclipse field, a check field was photographed both at Oxford 
and at 1 rincipe. ihe field chosen included Aj’ctiirus, so that it Avas easily fouiifl with 
the coelostat. Its declination was nearly the sanie 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 
fiom possible changes in the object-glass during transit. Unlike the .Sobral expedition, 
we were not able to take comparison j^hotographs 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 photograj^hed 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 puiq^ose, 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 unknowm scale. If, how^ever, a 
trustworthy scale is knoAvn.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 plades a determination of the scale of the Principe ’ 
photographs. 
32. All the measures w^ere made by Prof. Eddington wuth 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 w'-as possible because the Oxford plates 
were taken direct, and the Principe plates by reflection in the coelostat mirror. 
The small differences Ax and Ay, in the sense Principe —Oxford, Avere then measured 
for each star. Eight settings were made on each image ; for half of them the field w^as 
rotated through 180 degrees by the reversion prism. Five pairs of plates AA'ere 
measured, and the measures are given in Table XI. 
* ‘ Monthly Notices, R.A.S.,’ vol. LXI, p. 444. 
2x2 
