DETERMIXATIOX OF DEFLECTION OF LIGHT BY THE WUN’S GBAVITATIONAL FIELD. 309 
Subtracting the results of the comparison plates, so as to eliminate the errors 
arising from the intermediary scale plate we find for the displacements of the different 
stars, as compared wth those as given by Einstein’s Theory, with value 1"-15 at the 
sun’s limb :— 
No, of Star. 
Displacement in 
Right Ascension. 
Displacement 
in Declination. 
Observed. 
Calculated 
Observed. 
Calculated. 
11 
-0-19 
n 
-0-32 
// 
-1-0-16 
tt 
4-0-02 
5 
-0-29 
-0-31 
-0-46 
-0-43 
4 
-0-11 
-0-10 
+0-83 
+0-74 
3 
-0-20 
-0-12 
d-l-OO 
-bO-87 
6 
+0-10 
-hO-04 
+0-57 
-1-0-40 
10 
-O'08 
+0-09 
+0-35 
+0-32 
2 
-fO-95 
+0-85 
-0-27 
-0-09 
Photographs Taken 'with the Astrographic Object Glass. 
23. As stated above these photographs were taken with the astrograpLic object 
glass stopped down to 8 inches, mounted in a steel tube and fed by a 16-inch coelostat. 
From many years’ experience with the object glass at Greemvich it is certain that, when 
the object glass is mounted in a steel tube, the change of scale over a range of temperature 
of 10° F. should be insignificant, and the definition should be very good. It was realised 
that this high standard would not be obtained with the glass used in conjunction with 
the coelostat taken to Brazil, but nevertheless the results shown when the plates were 
developed were very disappointing. The images were diffused and apparently out of 
focus, although on the night of May 27 the focus was good.* Worse still, this change 
was temporary, for ’without any change in the adjustments, the instrument had returned 
to focus when the comparison plates were taken in July. 
These changes must be attributed to the effect of the sun’s heat on the mirror, but it is 
difficult to say whether this caused a real change of scale in the resulting photographs 
or merely blurred the images. 
The photographs were measured in the astrographic duplex micrometer, the eclipse 
photographs being directly compared with the comparison plates taken in July. All 
* The following note made at the time is quoted in full :—“ May 30, 3 a.m., four of the astrographic 
plates were developed, and when dry examined. It was found that there had been a serious change of 
focus, so that, while the stars were shown, the definition was spoilt. This change of focus can only be 
attributed to the unequal expansion of the mirror through the sun’s heat. The readings of the focussing 
scale were checked next day, but were found unaltered at 11-0 mm. It seems doubtful whether much 
can be got from these plates." 
2 u 2 
