46 STUDENTS HISTOLOGY 



Note the larger globules: they have become flattened by the 

 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 

 flattened air -bubbles among the oil -globules. Observe that these 

 air -bubbles have no intrinsic color, while the fat -globules are 



A | 



FIG. 23. 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 of C, D, thus giving but little color to the margin of 

 this globule. 



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 the pressure of the 

 cover- glass. 



MOVEMENT OF OBJECTS 



Objects are frequently seen moving in the field of the micro- 

 scope, the movement being magnified equally with their dimen- 

 sions. 



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 



