3.'?0 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. C. DAVIDSON ON A 



an extraneous determination of scale. In considering the programme it had seemed too 

 risky a proceeding, and it was thought that a self-contained determination would 

 receive more confidence. But this opinion has been modified by the very special 

 circumstances at Principe ; and it is now difficult to see that any valid objection can 

 be brought against the use of the scale. 



The temperature at Principe was remarkably uniform and the extreme range 

 probably did not exceed 4 during our visit including day and night, warm season and 

 cold season. The temperature ranged generally from 77^ to 79j in the rainy season, 

 and about 1 colder in the cool gravana. All the check plates and eclipse plates were 

 taken within a degree of the same temperature, and there was, of course, no perceptible 

 fall of temperature preceding totality. To avoid any alteration of scale in the daytime 

 the telescope tube and object-glass were shaded from direct solar radiation by a canvas 

 screen ; but even this was scarcely necessary, for the clouds before totality provided a 

 still more efficient screen, and the feeble rays which penetrated could not have done 

 any mischief. A heating of the mirror by the sun's rays could scarcely have produced 

 a true alteration of scale though it might have done harm by altering the definition ; 

 the cloud protected us from any trouble of this kind. At the Oxford end of the 

 comparison the scale is evidently the same for both sets of plates, since they were both 

 taken at night and intermingled as regards date. 



It thus appears that the checlc scale is legitimately applicable to the eclipse plates. 

 But the method may not be so satisfactory at future eclipses, since the particular 

 circumstances at Principe are not likely to be reproduced. As regards other sources 

 of systematic error, our chief guarantee lies in the comparatively large amount of the 

 deflection to be measured, and the test satisfied by the check plates that photographs 

 of another field under similar conditions show no deflections comparable with those 

 here found. 



V. GENERAL CONCLUSIONS. 



39. In summarising the results of the two expeditions, the greatest weight must be 

 attached to those obtained with the 4-inch lens at Sobral. From the superiority of the 

 images and the larger scale of the photographs it was recognised that these would prove 

 to be much the most trustworthy. Further, the agreement of the results derived inde- 

 pendently from the right ascensions and declinations, and the accordance of the 

 residuals of the individual stars (p. 308) provides a more satisfactory check on the 

 results than was possible for the other instruments. 



These plates gave 



From declinations 1 " 94 



From right ascensions . ... 2" -06 



