32 THE FROG 



the cover glass in such a way as not to soil it and, placing one edge 

 of it at the edge of the drop of water, let it fall so that it covers the 

 piece of paper. If this is done correctly there will be no air bubbles 

 imprisoned between the slide and cover glass and no excess of water. 

 Practice until you are successful, drying both slide and cover glass 

 between attempts. 



(e) Place the microscope squarely in front of you and adjust the 

 height of your chair so that you look down the tube comfortably. Do 

 not shift the microscope about after you get it ready for work. Have 

 the low-power objective in line with the tube and 3 to 5 millimeters 

 above the stage. Put the low-power ocular in position and open wide 

 the iris diaphragm. Now look down the tube, keeping both eyes open, 

 and move the mirror until you can see a circular area that is evenly 

 and brightly illuminated; this is the field of the microscope. Place 

 your mount of the piece of paper on the stage so that the paper is 

 directly over the center of the opening. As you look down it raise the 

 tube, by means of the coarse adjustment, until you see the lines 

 clearly. How far away do they appear to be? How large do the 

 millimeter squares seem to be? Move the slide to the right. Which 

 way did the lines move as you watched them? Learn to move the 

 object so that the image goes where you want it to go. Open and 

 close the iris diaphragm and observe how the illumination is altered. 

 Focus with the fine adjustment and observe the results. In which 

 direction do you need to turn the adjustment screws to move the tube 

 up; to move it down? 



(f) Follow carefully the steps in changing to the high-power 

 objective. Place a junction of two lines exactly in the center of the 

 field and focus sharply. Without changing the focus or the position 

 of the slide, rotate the nosepiece and thus bring the high-power objec- 

 tive in line with the tube. The lines will be seen indistinctly or not at 

 all. Focus carefully up through one turn of the fine adjustment screw. 

 If the lines do not come into focus reverse the turn and then focus 

 down through one turn of the fine adjustment screw. Try the change 

 from the low-power objective to the high-power objective several 

 times and learn which way the fine adjustment screw must be turned, 

 and how much, for your particular instrument. To master this oper- 

 ation now will save much time later. Do you find that you need to 

 open the iris diaphragm to see clearly with the higher magnification? 

 The low-power ocular will usually give you better results with the 

 high-power objective than the high-power ocular will. T"^nlcss in- 

 structed to the contrary, use the low-power ocular with the high-power 

 objective in subsequent exercises. 



