514 GENERAL BIOLOGY 



and M equals the magnification. Substituting the values 

 you have found / to have for your two lenses, determine 

 their magnification. 



4. The objective of the compound microscope is practi- 

 cally a simple lens. Take the shorter of the two objectives 

 in hand and examine with it the stippled background of a 

 printed halftone figure (such as figure 2, on page 8 in this 

 book). Then try the longer objective, and observe the 

 shortening of focal distance with increasing magnification. 

 Obviously, we soon reach the limits of practicability of 

 simple lenses. 



5. Hence, the compound microscope, with its eyepiece 

 for magnifying the already enlarged image produced by the 

 objective. Examine again the half-tone stippling with the 

 low power objective used as a simple lens and note the 

 apparent size of the stipple marks; (Better use for this and 

 the following a detached slip of paper printed in halftone, 

 for convenience in placing under microscope) then attach 

 this objective to the microscope, insert the longer (if there 

 be two) of the eyepieces in the top of the microscope tube; 

 place the stippling on the stage directly under the objective, 

 focus and again note the apparent size of the stippling. The 

 magnification of the compound microscope is determined by 

 multiplying that of the objective by that of the eyepiece. 

 Suppose that your objectives are of two-thirds inch and one- 

 fifth inch equivalent focus, and your eyepieces, of one inch 

 and one-and-one-half inch, respectively: what will be the 

 magnifying powers of the four possible combinations? 



6. Observe the effect of pushing in and pulling out the 

 draw tube of the microscope on the apparent size of the 

 object. The lenses of the usual laboratory microscope are 

 corrected for (and so, will give the best results with) a tub^ 

 length of 1 60 millimetres. 



