6 GENERAL BIOLOGY. 



III. Examine bubbles of air in water. To obtain 

 these, support a cover-glass by a thick piece 

 of filter-paper under one edge, run a drop 

 of water under the cover glass, and then tap 

 the glass. 



The cover-glass must be supported so that 

 the bubbles may be spherical. Sketch 

 (AA 2). (Keep for comparison with IV.) 



IY. Examine a small drop of oil suspended in water 

 with the cover-glass supported. Compare 

 the air and the oil, and note the differences 

 in the appearance of each upon focussing up 

 and down. Sketch. 



V. To make a scale. Find on a stage micrometer,* 

 with the low power, the lines ruled one 

 millimetre and one tenth of a millimetre 

 apart. Draw some of these as they appear 

 projected upon paper at the base of the 

 stand. While doing this keep both eyes 

 open and the head motionless. Draw sev- 

 eral sets of lines, and then transfer the most 

 accurate set to a piece of cardboard. 



Make a scale for the combinations AA 2, 

 D 2, and D 4 ; label each with the names of 

 lenses used, and the actual size of the mi- 

 crometer spaces. 



*Zeiss's Object-micrometer, No. 26, b. 



