210 



ELEMENTARY CHEMICAL MICROSCOPY 



therefore if suspended would subside only after a long period, it 

 is possible to weigh out a portion of the mixture, add it to water, or 

 better, water and glycerine, in a small graduated flask, fill to 

 the mark, shake well and quickly remove one cubic centimeter 

 or less, for counting. This method precludes an error arising 

 from non-uniform, quantities but is longer and more cumbersome 

 than the methods already described. 



FIG. 116. Object Slide Ruled in One-half Millimeter Squares. 



Instead of using a net ruled micrometer eyepiece some micros- 

 copists employ a slide ruled in squares or a tiny cell with ruled 

 bottom, as shown in Figs. 116 and ny. 1 The advantage of such 

 devices of permitting the use of any eyepiece is usually outweighed 

 by a number of undesirable features, chief among which may 

 be mentioned the objections that the rulings on the slides are 



FIG. 117. Girard Counting Cell for the Analysis of Flour. 



not always clear when the particles to be counted are in focus; 

 the relatively large size of the ruled squares with a high power; 

 and the difficulty of properly cleaning the slides without event- 

 ually injuring the rulings. 



When it is desirable to cover a definite area on the object 

 slide it is far better to employ a micrometer disk-diaphragm 



1 Made by Nachet et Fils, Paris, France. 



