DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 321 



The 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, l"-2 in the 

 y-co-ordinate. Further, the .r-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. f 



Plate X. 



(1) Comparison with Oxford Plate GI. 



The differential refraction for all the eclipse plates is 



a = 46-5, b, d= + 8-2, e= -27-0 



the differential aberration being zero. 

 For the comparison plate Gj 



= 19-1, b,d= -1-0-7, e= -28-3. 

 Hence for X Gj 



a = 21-4, (>,d= +7-5, e= +1-3. 



* Plate X has also the merit of a short exposure, 3s. We should mistrust the a'-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. 



f Of the comparisons of check plates, w t 6j 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 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. 



