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 sufficiently 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 - 2 t^ 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 



