274 USE OF THE MICROSCOPE 



avoid the air bubbles place one edge of the coverglass on the 

 glass slip first and then gradually lower the coverglass while 

 holding it between thumb and finger pressed against opposing 

 edges, and support it with a dissecting needle held under the 

 upper edge as the coverglass begins to flatten out the drop of 

 water. If now air bubbles are being entangled in the water the 

 coverglass may be rocked gently up and down with the dissecting 

 needles until the bubbles are broken and driven out. When 

 the coverglass is in position the water should fill out under it 

 to the edges, but no more. If it runs out over the slip the drop 

 was too big and the surplus should be wiped off with a piece 

 of filter paper or clean cloth. Take note in this first prepara- 

 tion whether the drop was too large or too small so as to avoid 

 the mistake in the future. The potato in the preparation should 

 now appear as a very thin film. If it is in a lump it may be 

 flattened out by pressing on the coverglass with the dissecting 

 needle. Now place the slip on the stage under the clips (see 

 the figure for the parts of the microscope as they are mentioned) 

 and bring the preparation over the center of the condenser or 

 to the center of the opening in the stage if there is no condenser. 

 With the plane mirror, if there is a condenser, reflect light from 

 the sky into the condenser and adjust the mirror so that the prep- 

 aration is seen to be illuminated while looking at it directly and 

 not through the ocular. Then with the rack and pinion coarse 

 adjustment run the body tube down until the lower power object- 

 ive, which has been revolved into position, is within a quarter 

 inch of the coverglass. Look in through the ocular and the field 

 of view should appear circular and brightly illuminated; if it is 

 not, adjust the mirror slightly until it is so. Now, still look- 

 ing through the ocular, slowly rack the tube back until the ob- 

 ject comes into view, and with the micrometer head of the fine 

 adjustment, turning to right or left as seems to be necessary, 

 bring the object into sharp focus. The torn and crushed tissue 

 of the potato will be seen and many very minute grains of starch 

 which need to be seen with the higher power objective. Ac- 

 cordingly draw the body tube up with the coarse adjustment 



