OBJECTIVES AND OCULARS 11 
with any fluid whatsoever a moment longer than absolutely 
necessary. 
When focusing a microscope upon a preparation, first turn the 
body tube down by means of the coarse adjustment until the 
objective is closer to the preparation than is indicated by the 
equivalent focus of the objective, watching carefully with the 
head to one side to see that the front lens is not forced against 
the slide. Look into the microscope and slowly raise the tube 
by the coarse adjustment until the object is almost in focus; 
complete the adjustment by means of the fine adjustment. 
Never focus down while looking into the instrument. Failure 
to observe this simple rule is apt to lead to serious loss and 
considerable expense. 
Never change from one objective to another without first 
making sure that the body tube has been raised sufficient!)^ to 
allow the new objective to be slipped into place without injury 
to the preparation on the stage or to the objective. 
Never handle objectives or oculars or, in fact, any parts of 
the microscope with dirty, greasy, or wet fingers, or when the 
hands are so cold as to incur danger of dropping the apparatus. 
Never use a high power until the preparation has first been 
examined and centered with a low one. Remember that it is 
possible to see more of the object and see it better with low 
powers than with high ones. 
Invariably work with the lowest power which will clearly 
define the preparation. The most common fault of the beginner 
is to employ too high a magnification. 
The initial magnification of an objective is the ratio of the 
equivalent focus of the objective to the optical tube length. 
For roughly approximate values we may calculate the initial 
magnification by dividing 250 by the equivalent focus, 250 
millimeters being the distance of most distinct vision of the 
normal human eye. An objective of 16 millimeters equivalent 
focus may therefore be considered to have an initial magni¬ 
fication of -W- or approximately 15. 
Oculars. — The function of the ocular or eyepiece of a com¬ 
pound microscope is to magnify the real inverted image of the 
