DETEKMINATION OF DEFLECTION OF LIGHT BY THE SUN’S GKAVITATIONAl. FIELD. 32 I 
The possibility of a determination of deflection practically depends on tlie appearance 
of star 5. The relative displacement of 5 and 3 is on Einstein’s theory, l"-2 in the 
y-co-ordinate. Further, the .T-measures of 5 are needed for a really good determination 
of the orientation. Star 11 can scarcely take its place. It is true that the relative 
displacement is then 0'’8 ; but the orientation affects this with a much larger factor, 
and the orientation is badly determined in the absence of star 5. 
Accordingly plates W and X are the only ones likely to give a trustworthy result. 
X is somewhat the better plate of the two.’’' Measures have been made of the faint 
diffused images on plates Y and Z ; but, as might have been expected, they are 
hopelessly discordant and cannot be reconciled by any adopted value of the 
deflection. 
35. We give the measures of plates X and W in detail. Both comparisons of X were 
measured at Principe a few days after the eclipse. Plate W, which was not developed 
until after the return of the expedition, was measured at Cambridge on August 22-23.t 
Plate X. 
(1) Comparison with Oxford Plate Oi. 
The differential refraction for all the echpse plates is 
a = — 46-5, 6, (^= + 8-2, —27-0 
the differential aberration being zero. 
For the comparison plate Gi 
—19-1, h,d=\Q-l, 28-3. 
Hence for X — Gi 
a =■ — 27 ■ 4, h, cl = -p 7*5, e = —1~ 1*3. 
* Plate X has also the merit of a short exposure, 3s. We should mistrust the r-measures of a long 
exposure with variable cloud and imperfect guiding, because there is nothing to show that the images of 
the different .stars are formed at the same time. 
t Of the comparisons of check plates, — G was measured on August 20, and the others about the 
end of September. Previous measures had been made at Principe with three earlier check jjlates taken on 
the night of May 16 ; but a slight change of adjustment of tilt was made the following day (thereafter 
it remained unaltered until the eclipse), and the small change of focus allowed for in the comparisons. 
These furnished a provisional scale which was used to obtain preliminary results. Afterwards the measure¬ 
ment of check plates was undertaken in a more systematic way, using later plates about which no doubt 
could arise, and giving the results printed above. No change of any importance was found ; the final 
value for the deflection at the limb was reduced by 0" • 4 compared with the provisional value, but this was 
mainly due to the adoption of separate values of a' and e' instead of adopting the mean, and to recalculation 
of the differential refraction and aberration. 
