MICROSCOPIC DRAWING AND ENGRAVING. 



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. 



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, when 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 object is'fixed, 

 54 



