114 ELEMENTARY CHEMICAL MICROSCOPY 



the same position. Coordinate movements are obtained by the 

 milled wheels T, T. The graduated scales in each instance are 

 supplied with verniers v, v. The object slide is held in position 

 by the fingers F, /; a spiral spring in the joint of F presses it 

 firmly against the corner of the slide. The screws a, a permit 

 changing the distance between F and /, thus providing for the 

 use of object slides or cells of different sizes. 



By means of the mechanical stage the investigator is enabled 

 to search systematically the entire area of a preparation in such 

 a manner as to ensure that no portion has been missed, nor has 

 any portion been twice examined, a matter of vital importance 

 in quantitative work, in clinical microscopy and in the examina- 

 tion of foods for adulteration. 



Before attaching a mechanical stage of this type to the micro- 

 scope, first lay a thin card or a piece of thick paper upon the stage 

 of the microscope, then lay the mechanical stage upon the paper 

 and securely clamp it in place about the base of the pillar of the 

 microscope. Pull out the card or paper and the stage is ready 

 for use. The card or paper has as its function preventing the 

 arms from rubbing upon the stage when the arms of the mechan- 

 ical stage are moved. Unless a tiny space is left between the 

 microscope stage and the mechanical stage, a free and smooth 

 movement of the preparation back and forth beneath the objec- 

 tive may be seriously hampered. 



In order that full use may be made of a mechanical stage the 

 amount of displacement must be indicated by equivalent scales 

 on each of the two movements. It is therefore essential to find 

 the value and the uniformity of the scale divisions and to find the 

 diameter of the field of the microscope as indicated on the scale of 

 the stage. This may be accomplished by laying a stage microm- 

 eter in place between the clips F/ of the stage and measuring 

 the displacement under a cross-haired eyepiece for different por- 

 tions of each of the lateral scales of the stage. There is thus 

 ascertained the true value of the graduations, whether both 

 scales are equivalent and whether the scale divisions are of uni- 

 form size. To determine the amount of stage displacement 

 necessary to just include an entirely new area, bring a line of 



