320 SIR F. W. DYSON, PROF. A. S. EDDINGTON AND MR. G. DAVIDSON* ON A 



The sign of the results shows that the scale of the photographs is larger at Principe 

 than at Oxford ; in fact the focus must have been set about 1-2 mm. further out 

 (apart from any change of length compensated by expansion of the photographic plates). 

 As the error in focussing was probably not more than 0-5 mm., the greater part of this 

 shift must be due to the focal length of the lens combination increasing with temperature 

 more rapidly than the linear expansion of the glass. 



If the only difference were a change of focal length, we should have a' e'. There 

 is a fairly strong indication that e' is greater than a'. This is no doubt due to a change, 

 in the definition caused by the coelostat mirror or by a shift of the object-glass lenses 

 on the journey ; and, as it will presumably affect the eclipse plates in the same way, 

 it is best to adopt the values of a' and e' as determined, rather than to take a mean. 

 In so doing we shall at any rate not exaggerate the displacement, which depends mainly 

 on the ^/-measures and is reduced by adopting too large a value of e'.* 



The difference b' - d' merely gives the relative orientation of the two plates as 

 placed face to face. The sum b' -\- d' practically vanishes, as it should do. However, 

 for consistency we adopt the small value found. 



From the internal discordances of our determination of e' (the most important of 

 these constants) the probable error of the mean is 2-1. This, as shown later, will 

 cause a probable error of our final determination of the deflection, reduced to the limb 

 of the sun, of amount 0"-14, affecting all determinations systematically. Errors 

 in the other constants have much smaller influence. 



The Eclipse Plates. 



34. The eclipse plates from K to S show no star images. After that the cloud 

 lightened somewhat, and some images appear on the 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 (KI and K 2 Tauri), which 

 theoretically should be strongly displaced, must be shown. They appear on all plates 

 from T to Z, and being near the centre 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, diffused. 

 * It happens that it is also reduced, but to a less extent, by using too small a value of a'. 



