150 ELEMENTARY CHEMICAL MICROSCOPY 



rulings of equidistant spaces with no provision for varying 

 the spaces or relative position of cross lines, and (b) those hav- 

 ing a scale consisting of a fixed or movable scale traversed by 

 cross-hairs displaceable by micrometer screws attached to the 

 graduated drums indicating the magnitude of displacement. 

 Micrometers of the latter sort are generally known as filar 

 micrometers and comprise the most accurate as well as most 

 convenient microscopic measuring devices in use. Only their 

 high cost prevents their more general employment. 



Since in micrometer oculars the graduated scale is so placed 

 as to fall in the same plane as that of the real image formed by 

 the microscope, the number of scale graduations covered by the 

 image gives a value for the size of the image only and not for the 

 object. It is therefore necessary in all cases 1 to first ascertain 

 the true value of the eyepiece scale with respect to each objective 

 used. This is accomplished by means of a stage micrometer. 



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 ad- 

 just 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. 93. It is obvious that in 

 such an event it is necessary to estimate with the eye what 

 fractional part of a division to add to the whole number 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 



1 An exception to this statement is to be found in ocular micrometers with 

 scales so ruled by the manufacturer as to yield with objectives supplied for use 

 with them a definite value. 



