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 loo 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 loo 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.^ 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; 
^ Made by Nachet et Fils, Paris, France. 
