428 Description of Mr. South’s 
very liable to be disturbed on reversion of the axis, and when 
the object-end of the telescope accidentally points below the 
horizon : also, after the index is set, should the position of the 
telescope be deranged before the observation is commenced, 
reference must be again made to the divisions of the semicircle ; 
and should the accident occur whilst the star is passing the 
wires, the observation will be lost. The apparatus to remedy 
these inconveniences is seen in Plate XVI. and XVII. figs. 1 
and 6 , but better in Plate XVI. fig. 2, which is drawn to a scale 
half the dimensions of the original. It consists of two complete 
circles, firmly attached to the eye-end of the telescope ; each 
circle is provided with two opposite verniers, subdividing its 
divisions into minutes of a degree ; the indices have clamps 
and slow moving screws, and microscopes are attached to the 
verniers : a spirit level is also affixed to the index of each 
circle, whose range of bubble corresponding to one minute, 
is about half an inch. When this apparatus is adjusted, on 
the vernier being set to the place of a star, and the telescope 
moved round till the bubble stand in the middle of its range, 
then will the star traverse the field between the two hori- 
zontal wires.* Hence it is evident, that should by accident 
the telescope be moved before, or during observation, the 
merely restoring the bubble to the middle of its range, will 
again present the star to the observer’s view, without any 
reference to the divisions. But it is often of importance to 
observe the transits of stars, one of which, in right ascension 
differs very little with the other ; as for instance, Capella and 
Rigel ; here the index of one circle may be set to the first 
star, whilst that of the other may be placed to the second ; 
* These wires are distant from each other, about four minutes of a degree. 
