of the Georgian planet. 301 
Measures taken with the micrometer may always be sup- 
posed to be accurate, unless they are marked as being affected 
by some circumstances existing at the time they were taken : 
when these are favourable, they can hardly be liable to any 
great error. 
The calculations which I have given with the observations, 
will show the appropriate confidence each of these three me- 
thods of obtaining the positions of the satellites may separately 
deserve. 
A much greater difficulty attaches to taking measures of 
distances than to those of angular positions : when the latter 
are taken, we have the position of the satellite in view all the 
time the planet passes along the parallel ; and, although the 
moment of ascertaining the angle is only that in which the 
planet is in the centre of the wires, yet a constant attention 
to the motion of the two bodies will sufficiently enable us to 
perceive any excess or defect in the parallelism between their 
situation and that of the adjustable wire, whereas in measures 
of distances, the telescope must be kept in motion to retain 
the two bodies in their contact with the two wires, which dis- 
turbance considerably affects the delicacy of vision, and more- 
over requires a divided attention, as the passage of each body 
over its respective wire must be viewed. The only excep- 
tion is, when the satellite is at 90 degrees, in which case the 
distance of the two bodies may indeed be measured with great 
accuracy. 
The lucid point micrometer which has been tried is subject 
to the same difficulties ;* its application to my construction of 
the 20 feet telescope, with regard to situation, is very conve- 
* For a description of this micrometer, see Phil. Trans, for 1782, page s 63. 
