DEFLECTIOlSr OF LIGHT DYSON AND OTHERS. 161 
were greatly inferior, and we decided to rely entirely on the 8-inch 
aperture. 
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 ccelostat. 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 ccelostat 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 expe- 
dition, 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 i^lates 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 meager 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 deter- 
mination of the scale of the Principe photographs. 
32. All the measures were made by Professor 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 coelostat 
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° by 
the reversion prism. Five pairs of plates were measured, and the 
measures are given in Table XI. 
8 Monthly Notices, R. A. S., Vol. LXI, p. 444. 
