190 Scientific Proceedings, Royal Dublin Society. 
Taste TV.—Dectuination 60°. 
| ‘ 
Hour Angie. a | b c d | ‘Zon. Dish 
1h —0:00001 0-00004 0-00007 0:00000 10°33’ 
2 -00001 -00008 -00015 -00000 17 34 
3 “00002 -00014 “00023 “00000 Oa aS 
4 “00004 “00022 “00032 “0000 32 24 
by) 00007 00033 "00043 -00002 39 27 
6 "00014 00048 00055 -00009 45 58 
7 “00024 -00065 -00067 “00021 51 51 
8 “00039 -00081 -00077 “00040 56 54 
9 -00061 “00090 -00079 “00069 61 2 
10 -00086 “00082 -00069 -001038 64 6 
11 -00107 -00050 “00041 “00133 65 59 
12 -00116 -00000 -00000 -00145 66 39 
The accompanying curves (figs. 1, 2, 3, 4) are traced with 
these quantities as ordinates, the hour angles being taken as. 
abscissee, and the ‘‘ Zenith Distance Curve” is added in each case- 
for convenience of reference. In the case of the B and D curves. 
the ordinates have been measured below the line of abscissee to. 
avoid confusion. From these curves, or from the tables them-- 
selves, it is easy to find the amount of the distortion which a star 
image undergoes when the exposure lasts from any given hour 
angle to any other. For instance, if a star at the equator is photo- 
graphed from the time when its hour angle is 4" to the time when 
its hour angle is 5", we find, in Table L., 
; | (2) (4) (¢) (d) 
Corresponding to 44, 0:00079 0-:00125 0-:00125 0-00148 
9 » OF, 003822 00468  -00468 -00596 
Difference, 0:00243 0:00343 0:00843 0:00453 
Now if we consider another star on the plate which differs in 
k. A. and declination from the guiding star by 1000”, or, which 
