- DEFLECTION OF LIGHT—DYSON AND OTHERS. 173 
validity, it reduces very considerably both accidental and systematic 
errors. The weight of the determination from the five stars with 
known scale is more than 50 per cent greater than the weight from 
the 12 stars with unknown scale. Its effect as regards systematic 
error may be seen as follows. Knowing the scale, the greatest relative 
deflection to be measured amounts to 1.2’’ on Einstein’s theory; 
but if the scale is unknown and must be eliminated, this is reduced 
to 0.67’... As we wish to distinguish between the full deflection 
and the half deflection, we must take half these quantities. Evidently 
with poor images it is much more hopeful to look for a difference 
of 0.6’’ than for 0.3’’.. It is, of course, impossible to assign 
any precise limit to the possible systematic error in interpretation 
of the images by the measurer; but we feel fairly confident that the 
former figure is well outside possibility. 
A check against systematic error in our discussion is provided by 
the check plates, as already shown. Its efficacy depends on the simi- 
larity of the images on the check plates and eclipse plates at Principe. 
Both sets are fainter than the Oxford images with which they are 
compared, the former owing to the imperfect driving of the ccelo- 
stat, which made it impossible to secure longer exposures, the latter 
owing to cloud. Both sets have a faint wing in declination, but this 
is separated by a slight gap from the true images, and, at least on 
the plates measured, the wing can be distinguished and ignored. The 
images on plates W and X are not unduly diffused except for No. 
10 on plate W. Difference in quality between the eclipse images and 
the Principe check images is not noticeable, and is certainly far 
less than the difference between the latter and the Oxford imayes; 
and, seeing that the latter comparison gives no systematic error in 
y, it seems fair to assume that the comparison of the eclipse plates is 
free from systematic error. 
The writer must confess to a change of view with regard to the 
desirability of using an extraneous determination of scale. In con- 
sidering the program 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—includ- 
ing day and night, warm season and cold season. The temperature 
ranged generally from 774° to 794° 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 
