Laboratory Equipment. 375 



of the hand holding the coverglass against the lower edge of the latter, 

 to keep it from sliding to the left ; gradually lower the coverglass, giving 

 it additional support by means of a dissecting needle held against its 

 lower surface. As it comes in contact with the fluid, see that no air 

 bubbles become entangled ; if there seems to be danger of this, briskly 

 move the coverglass up and down, by means of the dissecting needle, 

 until the bubbles become broken. The reason for lowering the cover- 

 glass gradually instead of simply dropping it over the object is to pre- 

 vent the formation of bubbles, which would make the preparation more 

 difficult to study. 



244. Manipulating the Microscope. Having properly mounted the 

 object, place the slide on the stage (Fig. 198, n) of the microscope so 

 that the object stands over the center of the opening in the stage. Set 

 the microscope in a convenient position (an upright one, when the ob- 

 ject is mounted in a thin fluid), and, by means of the concave surface 

 of the mirror, reflect diffuse light from a white cloud or other bright 

 portion of the sky (never use direct sunlight), so that the object is illu- 

 minated from below. This is done while looking directly at the object 

 outside the microscope. When the object is seen to be illuminated, 

 swing the f-inch objective into position, and rack the body of the 

 microscope down until the front face of the objective is within about 

 I inch of the object ; then look into the microscope and slowly rack 

 the body up until the object comes into focus. Finally, make the 

 image sharp by means of the micrometer-screw fine adjustment (g). 

 If the field does not appear bright on first looking through the micro- 

 scope, only a slight adjustment of the mirror is likely to be necessary. 

 If the image does not appear as the body of the microscope is drawn 

 upward, it can at least be told when the surface of the coverglass is in 

 focus, by means of flecks of dust which are quite certain to be present, 

 and the slide can then be moved slowly until the object is brought into 

 position. 



The potato pulp will appear as a more or less indistinct mass, but 

 free starch grains will be found suspended in the water. Adjust the 

 slide so that a group of the free grains is exactly in the center of the 

 field, rack the body of the microscope upward and swing the Hnch 

 objective into position, then rack downward until the objective nearly 

 touches the coverglass. In racking downward always observe the 

 objective to see that it is not forced against the coverglass. Now 

 look through the microscope and raise the objective by means of the 



