90 Farrants_, on a Micrometer Object-finder. 
glass slide. The convenience and aptness of tlie terms 
are a sufficient reason for continuing to employ them. 
Latitude refers to the position of an object with respect to 
the width of the glass. Longitude refers to its position with 
reference to the length of the glass. Latitude therefore will 
be determined by a line parallel to the sides of the slide which 
would pass through the object_, and the distance of such line 
from one or other of the sides of the slide_, expressed in terms 
of a scale that has been determined upon^ will denote the 
latitude of the object. The distance from one of the ends of 
the slip of a line parallel thereto^ expressed in like manner, 
will be the longitude. Latitude,, then^ will be measured by 
horizontal lines^ and longitude by vertical ones. 
Now, if the latitude of an object can be determined with 
precision, and if the line corresponding thereto can with ac- 
curacy be made to coincide with the transverse diameter of 
the field of view, the object itself may be brought into view 
by simply moving the glass slide along the stage. 
The ledge at the bottom of the object-plate, against which 
the slide rests when the body of the microscope is in an in- 
clined position, is a line from which the latitude may be con- 
veniently estimated, for as the lower edge of the slide rests 
against this ledge, the distance from it to the line on which 
the object is situate is the latitude required. For determining 
the longitude, it is convenient that the object-plate be for- 
nished with a stop on one side (preferably the left), against 
which the glass slide may be made to abut. This may con- 
sist of a narrow slip of brass, half an inch in length, pro- 
jecting a little above the surface of the plate, and having two 
pins on its under surface to drop into two small holes drilled 
in the object-plate to receive them. Such a stop is applied 
and removed in an instant, and aflPords a line from which the 
longitude may be measured, as certainly and as conveniently 
as latitude is measured from the ledge on which the slide 
rests. 
The scales to be used in determining the position of an 
object are ruled on a slip of glass, or other material, of the 
size ordinarily used for microscopic purposes, viz., 3 inches by 
1 inch. Let the centre inch be ruled with horizontal and 
vertical lines, at regular distances apart, say 50 to the inch ; 
the superficial square inch will thus be covered with small 
squares : this done, the observer is supplied with the means 
of accurately determining the position of any object. These 
scales are available at any moment, no preliminary arrange- 
ments are necessary before they can be used, and the readings 
being made under the microscope may be made with greater 
