HISTOLOGY 33 



Exercise 27. — Determination of the Magnification Obtained with 

 Different Combinations of Lenses. 



(g) In order to determine the actual sizes of objects studied with 

 the microscope it is necessary to know how many diameters the several 

 combinations of lenses will magnify. This can be determined as fol- 

 lows, using first the low-power ocular and low-power objective. Focus 

 sharply on the ruled lines of the preparation of millimeter paper, look- 

 ing with your left eye. At the same time, looking with your right eye 

 at your millimeter ruler on the table, measure one line and the space 

 between two adjacent parallel lines as they appear when projected to 

 the level of the table. This will seem difficult at first but can be done 

 quite accurately. Since the lines are actually 1 mm. apart the mea- 

 sured distance will give you the degree of magnification obtained, at 

 table level, with the low-power ocular and low-power objective of 

 your microscope. Make several measurements and compare your re- 

 sults with those of your neighbors, keeping a record for check by the 

 instructor. Repeat the process, using the high-power ocular instead of 

 the low-power ocular but with the low-power objective still in position. 

 Record your result for this second combination of lenses. 



(h) Place the low-power ocular in the tube again and focus with 

 the high-power objective. With this combination of lenses you cannot 

 see a complete square millimeter; you see a smaller area at a greater 

 magnification. In order to find out what the magnification actually 

 is measure the diameter of the field. Then, move the slide so that 

 one edge of the field coincides with the left edge of one of the ruled 

 lines. Notice a fiber of the paper which is conspicuous and which 

 coincides with the opposite side of the field. Move the slide until this 

 fiber reaches the edge of the field first occupied by the line. Repeat 

 this operation until you reach the left edge of the next ruled line; 

 measure one line and one space. Multiply the measured diameter of 

 the field as projected to table level by the number of times you shifted 

 the slide in passing from one line to the next. This will give you the 

 magnification with this combination of lenses. Insert the high-power 

 ocular and observe the fibers of the paper. The magnification with 

 this combination of lenses is very difficult to obtain by the method 

 previously used. It can be calculated with reasonable accuracy on 

 the basis of ratios if your determinations for other magnifications 

 were carefully made. Record the magnifications for all combinations 

 of lenses for use later. 



(i) At the close of each laboratory period leave the low-power 

 objective in line with the tube. Remove the slide from the stage; 



