158 ELEMENTARY CHEMICAL MICROSCOPY 



When employing the fine adjustment for micrometric meas- 

 urements, always make all movements in focusing in the same 

 direction, otherwise a serious error will be introduced due to 

 back-lash. 



If a piece of an object slide is used for calibrating the fine 

 adjustment, it must be remembered that we cannot focus first 

 upon the lower surface through the slide, then upon the upper 

 surface, to obtain its thickness, owing to the displacement of image 

 due to the higher refractive index of the glass than that of air. 

 This phenomenon enables us, however, to determine the thick- 

 ness of transparent objects when their refractive indices are 

 known by proceeding as described on page 200. 



Micrometric measurements by means of the fine adjustment 

 are often called for in chemical work, as, for example, to ascer- 

 tain the depth of corrosion, weathering, pits, streaks, etc., in 

 the surfaces of many different sorts of materials, or in approxi- 

 mating depths of penetration, or in measuring in transparent 

 bodies the displacement of images due to changes in refractive 

 index. This displacement enables one to calculate the refractive 

 index of the object. 



Measurement of Areas. The methods employed for the 

 determination of the areas occupied by microscopical objects is 

 discussed in Chapter X, page 216. 



