164 ANNUAL. REPORT SMITHSONIAN INSTITUTION, WW. 
Table XIII. — Check plates, plate constants. 
Comparison. 
Uncorrected. 
Corrected. 
6'. d'. e'. b'+d 
qi—ai- 
Wi—bi. 
S2-C1.. 
Ti—di. 
Di— ei-. 
+32.7 
+26.2 
+31.5 
+28.2 
+21.6 
+ 101.0 
- 16.0 
+192. 5 
+ 165.0 
- 76.2 
- 87.8 
+ 25.9 
-173.5 
-146.8 
+ 70.6 
+ 58.2 
+53.6 
+64.8 
+69.8 
+61.4 
+32.7 
+30.4 
+35.8 
+32.1 
+25.2 
+ 98.4 
- 22.5 
+ 182.6 
+ 157.8 
- 80.5 
- 90.4 
+ 19.4 
-183.4 
-154.0 
+ 66.3 
+32.1 
+31.4 
+42.1 
+45.0 
+35.7 
+ 8.0 
- 3.1 
-0.8 
+ 3.8 
-14.2 
Mean. 
+31.2 
+37.3 
1.3 
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 millimeters farther out (apart from any change of 
length compensated by expansion of the photographic plates). As 
the error in focusing was probably not more than 0.5 millimeter, the 
greater j^art 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 ccelostat 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 y measures 
and is reduced by adopting too large a value of <?'." 
The difference V — cV merely gives the relative orientation of the 
two plates as placed face to face. The sum V -\-d' practically van- 
ishes, 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 d=0.14''', affecting all determinations sj^stematically. 
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 
" It happens 
of of. 
that it is also reduced, but to a less extent, by using too small a value 
