of the Equatorial Instrument. ice) 
ever be the motion of the eye. When this point is obtained, the 
eye stop with its wires, must be there fixed, for that is their 
true place; viz. the correct focal point of the object glass; 
and whatever indistinctness may be found, from the diver- 
sity of eyes of different observers, must be corrected by the 
motion of the eye glass only. Another point to be secured is 
the permanency, as far as may be, in the position of the ob- 
ject glass ; for if this be not correctly centered, which is very 
rarely the case, and indeed never to be expected, that is, if its 
axis be not concentric with the axis of the cell, in which it is 
fixed, any motion of this latter, by screwing or unscrewing it, 
may not only change the place of the focus, to which the 
wires are adjusted, but will necessarily move the line of col- 
limation, both in right ascension and declination.* To ob- 
viate this, therefore, two corresponding marks should be made, 
with a graver, both upon the cell, into which the glass is bur- 
nished, and also upon the tube of the telescope, into which 
the cell is screwed, or otherwise inserted, that in case the ob- 
ject glass should ever be taken out to clean it, &c. it may 
be restored very nearly, if not exactly, to its former po- 
sition. 
The eye glass, object glass, and wires, being thus settled 
in their respective places to each other, it will not be an im- 
proper time to measure the interval between the wires, which 
cannot be too accurately done, being of such constant use ; 
this may be either, ist, by observing the passage of a star in 
the equator, and making proper allowances for the rate of the 
* Bv moving my object glass an entire revolution in its screw, the line of collima- 
tion appears to move through a little circle of 50" in. diameter, so that the eccentricity, 
in this instance, appears to have been about T g 5 inch, 
