92 The Microscope (chap. 9) 



2. Place the lamp so the diaphragm is about 10 inches from the micro- 

 scope mirror and line it up with the microscope so the light filament 

 centers on the plane surface of the mirror. Insert a neutral filter. 



3. Using a lOX ocular and 16 mm. objective, adjust the mirror until 

 the light passes through the substage condenser to the slide. Focus 

 microscope on the specimen and center the light. 



4. Rack up the substage condenser until it almost touches the slide and 

 close the substage diaphragm. Close the lamp diaphragm aboiu half- 

 way and adjust the lamp condenser until an image of the filament 

 is focused on the closed substage condenser diaphragm. This can be 

 observed in the microscope mirror or with a small hand mirror. 

 Partially open the substage iris diaphragm. 



5. Focus with the substage condenser until a sharp image of the lamp 

 diaphragm coincides with the specimen on the slide. With the mirror 

 center the diaphragm image. 



6. Take out the ocular eyepiece, and, looking down the tube, observe 

 the back lens of the objective. Open substage diaphragm until its 

 image edge almost disappears in the back lens, but remains just 

 visible. The back lens should be filled with light — full cone illumi- 

 nation. Because the light source is imaged on the diaphragm of the 

 substage condenser, the light source is in focus at the back focal plane 

 of the objective and the filament of the lamp can be seen. 



7. Replace the ocular eyepiece. If the tube length has to be adjusted, 

 do so (usually 160 mm. for American objectives). 



Certain precautions are imperative for good illumination and bear 

 repetition. The back lens should always be filled with even light and 

 the aperture of the substage diaphragm should be closed to a minimum 

 to furnish as wide a cone of light as the specimen will take. Closing 

 down the diaphragm to reduce light intensity and thereby increase 

 contrast in transparent objects is poor technique. Light intensity should 

 be controlled by the kind of bulb, use of filters, or a rheostat or variable 

 transformer inserted in the lamp system. 



Unless you have had previous experience with a certain microscope, 

 when changing from one objective to another of higher power (high dry 

 or oil-immersion) do not take it for granted that the microscope is 

 parfocal (focal planes of all objectives lie in same position on the tube). 

 The higher power may crush a valuable specimen. Proceed as follows: 

 examine slide with naked eye for approximate part to be examined. 

 Center the area over the stage aperture. Check with low power, then 

 middle power for the particular area of interest, then change to high 

 power, but raise the tube \ to \ inch at least before revolving the nose- 



