DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL 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 with those as given by EINSTEIN'S Theory, with value l"-75 at the 

 sun's limb : 



PHOTOGRAPHS TAKEN WITH THE ASTROGRAPHIC OBJECT GLASS. 



23. As stated above these photographs were taken with the astrograpliic 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 Greenwich 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 ccelostat 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 astrograpliic 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 



