202 
ELEMENTARY CHEMICAL MICROSCOPY 
essential. Rulings are usually of two types, as shown in Figs. 
124 and 125. Where type 124 is employed the entire field of 
view may be counted but in type 125 it is better to call a “ field ” 
that area comprised within the ruled square. This system is 
preferable to that of employing a cell with ruled bottom referred 
to below. An attachable mechancial stage will be found to be 
a great help in avoiding the making of counts in the same area 
more than once. 
Although the method just described appears at first sight to 
be crude and unreliable it has been found after a number of 
years’ trial in the hands of a large number of students to yield 
excellent results.^ 
In the case of starch mixtures, where the foreign component 
is present in the proportion of 3 to 7 per cent the results found 
are very close to the actual per cent, but when 7 per cent is 
reached, the beginner has trouble in obtaining reliable counts, 
and above 10 per cent the method requires great manipulative 
skill. 
It must, however, be borne in mind that a method of this 
sort even at its best gives merely a close approximation to the 
true value. 
The chief difficulties which will be encountered are those of 
removing equal amounts in every case upon the end of a tiny 
spatula; of obtaining a uniform distribution of the material 
throughout the drop; and of lowering the cover-glass upon the 
