42 AQUATIC MICROSCOPY FOR BEGINNERS. 



directly, but only by applying the drawing of the 

 magnified micrometer spaces to the drawing of the 

 magnified object. 



The eye-piece micrometer is a glass disk of such 

 a size as to slip into the eye-piece riiountingand rest on 

 the diaphragm always found there. The glass is ruled 

 in groups of parallel lines, the value of whose spaces 

 must be ascertained by the use of the stage-microme- 

 ter. 



With the latter on. the stage and focused, place the 

 eye-pieCe, carrying its micrometer,' in the body-tube 

 and rotate it until the lines on both micrometers are 

 parallel. Count the number of spaces on the eye- 

 piece micromster needed to fill exactly a single space 

 on the stage micrometer, and divide the known value 

 of that space by the number of spaces in the eye-piece 

 micrometer needed to fill it, and the quotient will be 

 the value of a single space of the eye-piece microme- 

 ter. Thus, if three spaces of the ocular micrometer 

 fill one of the y-jVo- inch spaces of the stage-microme- 

 ter, then each space on the former represents -gT^j-j- 

 inch. This process must be repeated with every ob- 

 jective, and the ocular micrometer must always be 

 used in the same eye-piece. 



With this, the object may be measured directly, by 

 counting the number of spaces it occupies in the ocu- 

 lar micrometer, and multiplying the ascertained value 

 of each space by that number. Thus an object just 

 filling three of the -g^i^ inch spaces of the eye-piece 

 micrometer would be xsAnr o'" tAt ^^ch in length. 



The stage-micrometer can also be used to ascertain 

 the power of the microscope. If each of the y-J-j- inch 

 spaces measures, when drawn on the paper, -^ inch, 



