76 JOURNAL OF PROCEEDINGS. 
Returning to the transit instrument. The transit instru- 
ment should be so placed that the telescope, if turned down to 
the horizon, points to the north and south, or the axis, H. B., 
be exactly directed east and west, then the vertical line de- 
scribed by the telescope must be the meridian. This, however, 
supposes three conditions to be fulfilled with absolute precision: 
Ist. The axis A. B. must be level. 
and. The line of collimation must be perpendicular to it. 
3rd. It must be directed due east and west. 
In the original construction and mounting of the instru- 
ment these three conditions are kept in view, and are nearly but 
cannot be exactly fulfilled in the first instance. In all astro- 
nomical instruments the conditions they are required to fulfil 
are only approximated to in the making and mounting, and a 
means of adjustment are in all cases provided, by which each 
of the required conditions, only nearly attained at first, are ful- 
filled with infinitely greater precision. In all such adjustments 
two provisions are necessary. Ist, a method of detecting and 
measuring the deviation from the exact fulfilment of the requi- 
site conditions, and secondly, an expedient by which such devia- 
tion can be corrected. 
Before proceeding with the principal adjustments just 
named, a minor one has to be performed, that is to obtain dis- 
tinctness of vision and parallax. The wires or spider lines 
should be in the common focus of the object-glass and eye- 
glass. In order to place them in the focus of the eye-glass, 
push in or draw out the eye tube until they are seen with per- 
fect distinctness. Now, if the wires are not in the focus of the 
object-glass when the telescope is pointed toward a distant 
mark, if the eye be moved a little to the right or left the mark 
will appear to move with reference to the wires. To remedy 
this the object glass or the wires must ba moved in the tube un- 
til the parallax is corrected. If after this adjustment the tele- 
scope in the meridian be pointed at a star, and the star allowed 
to run along the horizontal wire, and if it does not remain per- 
fectly bisected while the eye is moved up or down, the adjust- 
ment for parallax is not complete and must be gone over again. 
