16 
ELEMENTARY CHEMICAL MICROSCOPY 
objective is clean yet the field shows specks of dirt and appears 
blurred, the dust and dirt will be found to be on the disk carry¬ 
ing the cross-hairs or micrometer scale. Exceeding great care 
is required in cleaning cross-hairs and micrometer plates resting 
upon the diaphragm of the ocular and should be undertaken 
only by a person having patience, care and steady nerves. 
Use low oculars first and confine the work whenever possible 
to medium powers. Have recourse to high-power oculars only 
as a last resort, since they cut down the light to such an extent 
as to cause fatigue and eye-strain. 
Always look into a microscope with both eyes open. 
In the study of flat preparations between sfides and cover 
glasses, the general rule is to obtain the proper magnification 
chiefly by means of the objective, using a low-power ocular. But 
in the case of irregular surfaces or curved and heaped-up drops 
of liquid, the reverse is essential and low-power objectives (having 
long free working distance) and high oculars must be adopted. 
The latter procedure is also indicated when employing dark- 
ground illuminators or ultra-condensers, namely, increase the 
magnification by the ocular. 
Limit of Magnification. — A consultation of the tables of 
magnification given in the catalogues of the leading makers of 
microscopes and microscope lenses will show that with the mod¬ 
ern compound microscope employed in the usual manner with 
stock achromatic objectives and Huygenian oculars, a magni¬ 
fication as high as 1500 to 2000 may be obtained, and that with 
stock apochromatics and compensating eyepieces this may still 
further be increased to 3000, the upper limit of fisted combi¬ 
nations. 
Theoretically there is no limit to the magnification which 
may be obtained. But this must not be confused with resolving 
power which enables us to see things clearly and permits differ¬ 
entiating one part or structure from another. Great magnifica¬ 
tion avails us nothing if the image be blurred and irrecognizable. 
A little thought will show that there must be a limit to the 
resolving power practically available beyond which we can¬ 
not go. 
