DETERRflNATION 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 y-comjiarisons 
are free from any serious systematic error. 
Star 7 is of particiflar interest ; its position near the centre of the held corresponds to 
that of K-^, Ko Tauri in the eclipse field, from which the greatest fleflection 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 Kj 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 and k, 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 photograjrhs 
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 tlie latter 
case can only be attrilmted 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, h, cl, 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 plade-constants, ci', h', 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. 
Comparison. 
Uncorrected. 
Corrected. 
a. 
b. 
d. 
e. 
a'. 
h'. 
d'. 
e'. 
b'+d'. 
?! - «i 
+32-7 
+ 101-0 
- 87-8 
+58-2 
+32-7 
+ 98-4 
- 90-4 
+32-1 
+ 8-0 
— &J, . 
+26-2 
- 16-0 
+ 25-9 
+53-6 
+30-4 
- 22-5 
+ 19-4 
+31-4 
- 3-1 
^2 ^1 
-1-31-.5 
+ 192-5 
-173-5 
+ 64-8 
+35-8 
+ 182-6 
-183-4 
+42-1 
- 0-8 
H - ffi 
-B28-2 
+ 165-0 
-146-8 
+69-8 
+32-1 
+ 157-8 
-154-0 
+45-0 
+ 3-8 
H H 
-f2D6 
- 76-2 
+ 70-6 
+61-4 
+25-2 
- 80-5 
+ 66-3 
+35-7 
-14-2 
Mean. 
+31-2 
— 
+37-3 
- 1-3 
