DEFLECTION OF LIGHT — DYSOIiT AND OTHERS. 165 
remaining plates. The sky was never clear and nothing fainter 
than 5.5"" is shown. The cloud was variable in different parts of the 
plate, so that the brightness of the images varies erratically and the 
diffusion is also variable. 
In order to obtain results of any weight the stars 4 and 3 (y.^ and 
^2 Tauri), which theoretically should be strongly displaced, must be 
shown. They appear on all plates from T to Z, and being near the 
center of the field have good images. They are relatively rather 
faint on plate U, but are bright on the other plates. The appearance 
of the remaining stars is as follows : 
Plate T.— 6 bright ; 10 faint. 
Plate U. — 6, 10 very bright : 11 faint 
Plate v.— 6 bright ; 10 fair. 
Plate W.— 5, 6 good ; 10 diffused. 
Plate X.— 5, 6, 11 good. 
Plate Y.— 5, 6, 11 faint, diffused ; 12 very faint. 
Plate Z.— 5, 6, 11 faint, diffusetl. 
Tlie possibility of a determination of deflection practically depends 
on the appearance of star 5. The relative displacement of 5 and 
3 is on Einstein's theory, 1.2'' in the y-coordinate. Further, the 
a?-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 miages on plates Y 
and Z; but, as might have been expected, they are hopelessly dis- 
cordant and can not be reconciled by any adopted value of the de- 
flection. 
35. We give the measures of plates X and W in detail. Both com- 
parisons 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." 
^» Plate X has also tlie merit of a short exposure, 3 seconds. We should mistrust the 
<p-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. 
" Of the comparisons of checlc plates, Wi — &i was measured on August 20, and the others 
about the end of September. Previous measures had been made at Principe with three 
earlier check plates 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 fui-nished a provisional 
scale which was used to obtain preliminary results. Afterwards the measurement 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. 
12573°— 21 12 
