174 HOW TO SEE WITH THE MICROSCOPE. 



examined on the stage. With a one and one-half or 

 two-inch objective (having raised the prism so as to 

 contact nearly the under surface of the slide), focus 

 entirely through the body of the prism, so that its lower 

 edge may be seen in the field, and by turning the prism 

 in its mounting make this line, by estimation, as nearly 

 vertical as possible. If, perchance, this line appears to 

 the eye considerably to the right or left of the centre 

 of the field there is no harm done ; but you must make 

 a note of the fact; this would indicate, however, that 

 either the stage is out of centre, or the mounting of 

 the prism has been injured. 



Next, rack back the objective and remove the slide, 

 place a drop of glycerine on the top face of the prism ; 

 replace the slide and raise the prism so as to make con- 

 tact with its under surface. Having made this contact 

 exactly, depress the prism, say, about one-fiftieth of an 

 inch. Focus again with the low power, and bring the 

 lower edge truly vertical, as before instructed. 



Remove the low power and substitute the wide-aper- 

 tured objective, and by way of illumination provide a 

 small kerosene hand lamp, the flame of which ought not 

 to be higher than two-thirds the distance from the table 

 to the stage of the microscope; remove, also, the sub- 

 stage. 



Now, if the lower edge of the prism was seen to the 

 right of the field place the lamp to the right of the 

 stage ; on the contrary, if the edge was seen to the left, 

 pla,ce the lamp to the left ; and in either case swing the 



