92 
FarrantSj on a Micrometer Object-finder. 
tlie field of view with J-incli object-glass will include nearly 
4; or if any square be brought to the centre of the field 
there will also be visible the half of 4 squares^ one on each 
side of the central one, and nearly a quarter of 4 others_, 
one at each corner of that which occupies the centre of the 
field. 
The mode of using these scales is as follows :• — If, for 
example, it be wished to register the latitude of an object in 
the field of the microscope (the lower edge of the glass slide, 
it is presumed, resting against the ledge of the object-plate), 
bring the object as nearly as possible to the centre of the 
field ; then remove the slide from the stage, and put in its 
place the ruled slip ; observe which line coincides with the 
transverse diameter of the field, that is, with the position of 
the object, then count (upwards, as it appears in the micro- 
scope) from this line to the end of the series : suppose the 
45th line is that which corresponds with the position of the 
object, then 45 will denote the latitude of that object, that 
is, its distance from the lower edge of the slide. At any 
future time, and with any instrument, this object can be 
readily found; to effect this, place the ruled slip on the 
object-plate, the lower edge resting evenly against the ledge, 
then bring the upper line (as it appears) of the horizontal 
series into view, and count 45 from it (this is easily and 
quickly done by taking advantage of the conspicuousness of 
every fifth line), make that line coincide with the transverse 
diameter of the field of view; the ruled slip may then be 
removed from the stage, and the slide containing the object 
be put in its place ; the object sought now lies somewhere on 
the line which corresponds with the transverse diameter of 
the field, and if the glass be slowly moved across the stage 
will soon come into view. 
It is generally sufficient to determine with exactness the 
latitude of an object : if, however, it be thought desirable to 
register the longitude also, this may be done by proceeding 
in a manner altogether analogous to that just described, the 
only difference being that it is the position of a vertical line 
passing through the object that is now to be ascertained, and 
its distance from the end of the slide. Now it is that the 
lateral stop will be found convenient ; it is not, however, in- 
dispensable, since a temporary expedient may be made to 
supply its place, or the lateral edge of the object-plate may 
be taken as the line from which the longitude is measured ; 
in that case, the end of the slide containing the object must 
be brought even with the edge of the object-plate, and the 
object being brought into view by means of the lateral stage 
