62 MANIPULATION OF APPAEAT I X 



Air Bubbles. 



Take a clean slide and place in the center a few drops of muci- 

 lage or glycerin. Beat this with a knife until it has a frothy appear- 

 ance due to the presence of numerous air bubbles. Cover with a 

 cover-glass and focus on a small bubble. If the rays of light from 

 the mirror pass perpendicularly through the slide, the bubble will 

 have a light center and a uniform dark border. If the mirror is 

 not central, the light spot will be at one side of the center. After 

 taking off the sub- stage attachments, move the mirror bar until it 

 is at an angle with the stage, and observe, when the light passes 

 through the bubble, the direction from the center which the light 

 spot has taken. It will be away from the side to which the mirror 

 bar was moved. 



Now beat some cedar oil or oil of cloves, and repeat the experi- 

 ment. Observe the direction taken by the light spot in the oil 

 globule. It will be contrary to that taken by the one in the ah* 

 bubble, or to the side on which the mirror bar was swung. 



It is interesting to mix the mucilage with the oil and find an 

 oil globule and air bubble side by side. Then study the effect of 

 oblique light to identify the bubble or globule. 



It is advisable that the student familiarize himself with the 

 more common objects that he will meet, perhaps, as foreign 

 bodies in his studies. Mounts should therefore be made on a 

 slide dry or in a drop of water of such objects as spores, dust, cot- 

 ton and woolen fibers, hair, cork, etc., and studies made of them 

 with the high and low powers. 



