78 JOURNAL OF PROCEEDINGS. 
and if on moving the telescope in altitude the mark is perfectly 
bisected by the central wire from top‘to bottom this wire is 
. perpendicular to the horizontal axis A. B., if not the ring or 
tube containing the wires must be turned around until the mark 
is bisected by all parts of the wire. 
To make the line of collimation perpendicular to the axis, 
it must be remembered that the line of collimation is a line 
drawn from the centre of the object-glass to the intersection of 
the middle wires in the field of view of the telescope. The cen- 
tre cf the object-glass is fixed relatively to the telescope, but the 
wires are so mounted that the position of their intersection can 
be moved through a certain small space by means of screws. 
One end of the line of collimation therefore being movable, 
while the other is fixed, its direction may be changed at plea- 
sure within limits determined by the construction of the eye- 
glass and the screws. ‘This line of collimation is called the 
principal sight line. 
To ascertain whether the line of collimation is or is not at 
right angles to the line joining the points of support, A and B, 
let any distant point be observed upon which the intersection of 
the wires falls. Let the instrument be then reversed upon its 
supports, the end of the axis which rested on a being transtfer- 
red to b, and that which rested on D to a, and let the same object 
be observed. I it still coincides with the intersection of the 
wires, the line of collimation is in the proper direction, but if 
not, its distance from the intersection of the wires will be twice 
the deviation of the line of collimation from the perpendicular, 
and the wires must be moved towards the object through half 
of its apparent distance from it. 
To render this more clear, let A B represent the direction 
of the axis, C D that of a line exactly at right angles to it, or the 
direction which is to be given the line of collimation, and let 
C D represent the erroneous direction which that line actually 
has. Let S bea distant object to which it is observed to be di- 
rected, this object being seen upon the intersection of the wires. 
If the telescope be reversed the line C D will have the direction 
C D, deviating as much from C D to the right as it before de- 
