78 
PROFESSOR W. N. HARTLEY AND MR. W. E. ADENEY 
Table of means and numbers adopted as correct measurements of the fiducial lines 
on each plate (continued). 
Spectra to the right. 
Portions 11-18. Measurements in hundredths of an inch. 
(i) 
Fiducial line?. 
Magnesium 
measurements. 
Adopted numbers. 
Intervals. 
I. 
o-ooo 
o-ooo 
101-475 
II. 
101-53 
101-475 
96-475 
III. 
198-00 
197-95 
91-625 
IY. 
289-54 
289-575 
86-925 
V. 
376-50 
376-50 
82-375 
VI. 
458-905 
458-875 
(2) (3) 
Fiducial lines. 
Thallium 
measurements. 
Adopted 
numbers. 
Intervals. 
Zinc 
measurements. 
Adopted 
numbers. 
Intervals. 
I. 
o-ooo 
0-000 
101-65 
0-000 
o-ooo 
101-70 
II. 
101-66 
101-65 
96-65 
102-14 
101-70 
96-70 
III. 
198-63 
198-30 
91-80 
198-965 
198-40 
91-85 
IY. 
290-175 
290-10 
87-10 
290-22 
290-25 
87-15 
V. 
377-27 
377-20 
82-55 
377-43 
377-40 
82-60 
YI. 
459-76 
459-75 
459-95 
460-00 
It will be observed that there are differences in the measurements of the fiducial 
lines which cannot be accounted for by errors of observation. These differences may 
be due to two causes : first, the difficulty of placing every photographic plate in exactly 
the same position in the camera; second, to the alteration in length of the screw of 
the dividing engine by change of temperature when the different plates were measured. 
That the temperature has some effect upon these measurements appears probable from 
the fact that the first dozen or so of photographs which were measured on consecutive 
days give those readings which agree most closely. The dividing engine could not be 
kept always in the same room, and on its removal it was probably subjected to small 
