182 



ELEMENTARY CHEMICAL MICROSCOPY 



objective used. This is accomplished by means of a stage microm- 

 eter. 



Focus the eye lens of the ocular so that the graduations of the 

 ocular scale become clear and distinct. Lay the stage microm- 

 eter upon the stage and move it until the center of the rulings 

 falls in the optic axis of the microscope, focus carefully and 

 adjust the micrometers by turning ocular or stage or both until 

 the rulings in one scale are parallel to those in the other. Move 

 the stage micrometer until a line becomes coincident with a line 

 of the ocular scale. Count the number of divisions of the ocular 

 scale included between one or more divisions of the stage microm- 

 eter. Divide the value of the stage scale by the number just 

 obtained. The quotient equals the true value of one ocular 

 scale division. It is usually the case that conditions obtain 

 giving an appearance shown in Fig. 115. It is obvious that in 



such an event it is nec- 

 essary to estimate with 

 the eye what fractional 

 part of a division to 

 add to the whole num- 

 ber of divisions of the 

 ocular scale included in 

 one division of the 

 stage micrometer. Such 

 an estimation or guess 

 introduces a serious 

 error into our method. 

 Moreover, the image of 

 an object to be meas- 



Micrometer .Scales Improperly Adjusted. ^ 



actly a whole number 



of divisions of the ocular micrometer and we are obliged to 



make a guess as to what fraction of a part to add. Thus there 



are two estimates necessary and any measurements recorded 



must necessarily be mere approximations. The second of these 



errors cannot be eliminated in micrometer oculars with fixed 



scales having rulings of non-variable magnitude, but the deter- 



FlG 



"5- 



