'36 



PRACTICAL MICROSCOPY. 



is a spheroid. These bodies are fat globules. Particles of free fat 

 always assume the spheroidal form when suspended in a liquid. 



Note the larger globules; they have become flattened by pressure of 

 the cover-glass. 



Clean the slide, and make a second preparation from the diluted 

 milk first, however, shaking it violently in a bottle. Note the flat- 

 tened air bubbles among the oil globules. Observe that these air 



A| 



FIG. 20. DIAGRAM SHOWING "THE EFFECT OF AIR BUBBLES AND OIL GLOBULES IN A MOUNTED 

 SPECIMEN UPON THE RAYS OF LIGHT. 



The lines A, B, show the refraction of the rays (so as to produce a ring of color) by the action 

 of two plano-concave water lenses which are formed by the air-bubble. 



The oil is seen to correct the refraction cf C D, thus giving but little color to the margin of 

 this globule. 



bubbles have no intrinsic color, while the fat globules are faintly 

 yellow. Observe the change in the ring of prismatic color about the 

 edge of the air bubble, as the focus is altered. No such color will be 

 .seen in connection with the oil globule. 



The bubbles assume various figures from pressure of the cover glass. 



MOVEMENT OF OBJECTS. 



Objects are frequently seen moving in the field of the microscope, 

 the movement being magnified equally with their dimensions. 



Thermal Currents. When with the previous specimen, or any 

 other fluid mount, the warm hand is brought close to one side of the 

 stage, the globules in the field will be seen swimming more or less 

 rapidly. These currents are due to the unequal heating of the liquid 

 under observation. The direction of the current is in the reverse of 

 its apparent motion. 



Brownian Movement. Place a fragment of dry carmine on a slide; 

 add a drop of water, and with a needle stir until a paste is formed. 

 Add another drop of water, and immediately put on the cover- glass. 

 With H, note the most minute particles, and observe their peculiar, 

 dancing motion. This occurs when almost any finely-divided and 

 generally insoluble solid is mixed with water; it ceases after a short 

 time. The movement has been attributed to several causes. 



