MICROSCOPIC DRAWING AND ENGRAYING. 
12. Scales are sometimes engraved so as to indicate at once the 
dimensions of an object in length and breadth, by lines dividing 
Fig. 4. 
Fig 5. 
the glass in directions at right angles one to the other, as shown 
in fig. 4, upon a greatly magnified scale. 
13. The dimensions of a minute object are sometimes ascer¬ 
tained by a somewhat different 
expedient. 
Let two lines, a a ' and b b'\ 
fig. 5, intersecting at right 
angles, be engraved upon a slip 
of glass, which can be inserted 
into the tube of a microscope., as 
shown in figures 6 and 7, through 
an opening in the side, which 
can be closed when such measure¬ 
ment is not required. These 
engraved lines, w T hen the micro¬ 
scope is properly adjusted, will 
be seen like two fine threads pro¬ 
jected on the object, as shown in fig. 5. 
Arrangements are made by which, while the Aject is fixed, 
54 
