74 JOURNAL OF PROCEEDINGS. 
small mirror does not interfere with the rays from the object 
glass. The instrument can also be reversed upon its rotation 
axis to eliminate instrumental errors, that is, to make the east 
and west ends of the axis change places. 
The principal sight line is marked by the centre thread or 
wire of the reticule, and is a line drawn from the middle of that 
thread through the optical centre of the object glass. This ~ 
line is technically called the line of collimation. The optical 
axis of a lens is the line which joins the centres of the spherical 
surfaces of the lens, and in a telescope properly constructed the 
axis of the object-glass and eye-glass should lie in the straight 
line joining the centres of the object-glass and eye-glass. This 
line is called the optical axis of the telescope. 
Before going further in the description of the transit in- 
strument I will digress a little and tell you of the instruments 
for measuring time. There are several mechanical contri- 
vances that are now in general use for measuring time, viz., 
sun dials, watches, chronometers and clocks. Of these the 
former, simply by means of a gnomen placed at the proper 
angle, throws the shadow of it cast by the sun on to a dial 
marked with hours sub-divided generally to five minutes. This 
gives apparent solar time, and of course can only be read while 
the sun is shining. 
Chronometers are very perfect watches, with a compensa- 
tion balance wheel to overcome the changes of temperature. 
They are usually of two kinds, Solar and Sidereal, beating half 
seconds. In some chronometers an electrical break circuit 
attachment is provided, to be used in connection with a chrono- 
graph. 
The ordinarv every-day clock in use records mean solar 
time, but for astronomical work a better and more perfect clock : 
is needed. The very best workmanship and material is put 
into its construction and the best of care and attention given it. 
It is provided with a seconds compensated pendulum, by which 
the effect of changes of temperature is better eliminated than 
in chronometers. There are two forms of pendulums in use, 
the Harrison, or gridiron, and the mercurial pendulum. The 
