36 LABORATORY DIRECTIONS 



4. Place wool fibers on a clean slide. Apply coverglass and 

 study when dry and when wet as for cotton. Examine the out- 

 line of the fiber carefully. Draw a wet fiber, 10 mm. in diame- 

 ter, and construct a cross-section as directed before. 



What differences do you observe between the dry and the 

 wet fibers? Notes required. 



5. Mount a drop of a Protozoan culture in such a way as 

 to include minute air bubbles. The inclusion of cotton fibers with 

 the drop of water will facilitate the formation of the air bubbles. 

 Study air bubbles and Protozoa and other micro-organisms with 

 special reference to the use of the focusing and lighting adjust- 

 ments of the microscope. No drawings are required. 



6. To determine the size of microscopic objects. One of 

 the commonest means of measuring microscopic objects is by 

 means of an OCULAR MICROMETER. The oculars for which these 



Figure 16. The use of an ocular micrometer. A cell under the low 

 power of the microscope. Note that the cell is 14 of the smallest 

 micrometer divisions in length. Its actual length is then; 

 14x0.0078 mm. 0.109 mm. Since the scale in the ocular remains the 

 same regardless of the objective used this same cell under high 

 power would be about 64 of the smallest micrometer divisions in 

 length. Consequently each division would have a correspondingly 

 smaller value. 



directions are given have a scale divided by fine lines into 100 

 units of length. The lines appear in the field of the microscope 

 at the same time that the object to be measured is seen. Notice 

 that the upper portion of an ocular micrometer may be pulled 

 out or shoved in to permit of sharp focusing upon the ruled scale. 

 Before inserting the ocular micrometer into the tube of the mi- 



