14 Mr. Herschel's and Mr. South's observations of the apparent 
from one end to the other, backwards and forwards, by 
the right ascension motion of the instrument ; but being once 
well adjusted, was found liable to no change, except in one 
instance, when the micrometer had received a blow, the defect 
produced by which was immediately discovered and rectified. 
Respecting the precautions used in observing, a few words 
will suffice. In measuring distances, the stars were bisected 
by both wires, and kept on them by means of the long handle 
of the slow R. A. motion, held in the left hand, and gently 
turned between the finger and thumb, the right being at 
liberty to manage the micrometer. This, though rather diffi- 
cult at first, becomes easy by a little practice, and even in 
unfavourable positions, the effect of the earth's diurnal motion 
may be almost exactly neutralised with a little management. 
The measures of distance are therefore all central , a circum- 
stance the more necessary to be noticed, by reason of the 
greater size of the spurious discs of stars in refracting, than 
in reflecting telescopes. In taking angles of position, these 
spurious discs are often extremely troublesome, as their 
inequality renders it very difficult ( especially in close stars ) , 
to judge of the position of the line joining their centres. In 
such cases a green, or even a slightly smoked glass, was 
sometimes used in viewing bright stars, or advantage taken 
of the favourable intervention of a thin cloud, which reduces 
them to mere points, or even of broad daylight, to obliterate 
their rings and scattered light, &c. Such cases are noticed 
when they occur, but it may not be amiss to mention, that the 
angle of position of a pair of very close stars, or very unequal 
ones, at a moderate distance, (such as e Bootis, (3 Orionis, &c.), 
can never be obtained with any degree of certainty by a single 
