QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 205 



of the gum-glycerine liquid. Portions of this suspension are 

 placed upon slides and the number of characteristic elements 

 per ioo Lycopodium spores is ascertained by count and com- 

 putation. The powdered material of unknown percentage com- 

 position which contains the same constituents is treated in an 

 exactly similar manner and the number of characteristic elements 

 per ioo Lycopodium spores is determined. The two ratios thus 

 obtained are directly proportional to the percentage composi- 

 tions. The results published by Wallis indicate that the method 

 is capable of great accuracy and may be regarded as much more 

 than an approximation. 



Fig. 127. 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. 127 and 128. 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. 128. 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; 



1 Made by Nachet et Fils, Paris, France. 



