18 
ELEMENTARY CHEMICAL MICROSCOPY 
copist cannot hope in practical work to carry the resolving power 
of his instrument beyond this limit. 
In ordinary work a magnification of from 750 to 900 diameters 
is the upper limit of true usefulness in the study of details of 
structure. Above this point the worker must be an exceptionally 
keen and skillful observer in order that he may properly interpret 
the appearances seen in the images formed. 
It is best, therefore, to make it a rule to work with low magni¬ 
fications. 
Study the preparation thoroughly and have recourse to high 
powers only when absolutely necessary. The dangers of errors 
of interpretation are thus greatly reduced and fatigue and eye 
strain practically eliminated. Moreover, it must never be for¬ 
gotten that with high-power objectives a very small area only 
is visible and the relation of the structure of the tiny area in 
the field of view to that of the adjacent areas bounding it may 
often be overlooked or be only imperfectly understood. Faulty 
deductions are apt to follow. 
The student will find that when using a Bausch & Lomb 
chemical microscope and medium power eyepiece (7.5 X) the 
diameter of the circular area visible with a 32 mm. objective 
is approximately 4 mm.; with a 16 mm. objective the visible 
area is reduced to a circle about 1.9 mm. in diameter; with an 
8 mm. objective the tiny circular area is only 0.68 mm. in diam¬ 
eter, while with a 1.9 oil immersion the circular area visible 
is less than 0.2 mm. in diameter. 
Use low-power eyepieces whenever possible. In the study of 
objects mounted between object slides and cover-glasses, obtain 
increased magnification by higher powered objectives, but with 
uncovered objects, drops of liquid and in microscopic chemical 
analysis it is best to obtain increased magnification by employ¬ 
ing higher powered eyepieces. 
In microscopic qualitative chemical analysis employ low-power 
objectives which have been specially selected, if possible, because 
of their long working distance and high penetrating power; 
sacrifice resolving power for the convenience of being able to 
thus obtain great depth of focus. 
