Lab 6 : Specimen Preparation for Microscopy

One student should cut an 18 mm long piece of metal out of bolt, and prepare it for micro examination of its transverse section by polishing and etching.

Show your specimen to the instructor at this stage. If your specimen is in proper condition for proceeding, he will direct you to continue and may demonstrate the polishing technique to use.

Polish the specimen.

After completing the polishing, show your specimen to instructor. He will check its finish and indicate whether you may proceed to the etching step. He may also demonstrate etching techniques.

Etch with 2% nital.

Ask the instructor to check your etched specimen and advise you whether to proceed to examine it on the microscope or whether you need to again polish and re-etch.

Examine the microscope. Locate each component identified by name and complete the following table by listing the magnifying power of each objective lens and each eye piece. Indicate the total magnifications available on microscope.

Use coarse adjustment wheel to lower the stage (or raise body) so that there is sufficient space between the objective lens and the stage for specimens to fit without coming too close the lens.(There should be sufficient clearance so that there is little likelihood of scratching the lens or specimen when the specimen placed on the stage.)

Put the lowest total magnification lens combination (objective and eyepiece) into position on the microscope. Turn on the microscope light.

Place one specimen on a putty-covered glass slide. Place the slide directly under the objective lens being used, trying to center the specimen under the lens. Do not look through the microscope yet. Instead, watch the etched surface of the specimen by directly looking at it. While watching the specimen, turn the coarse adjustment to slowly raise the stage towards the objective lens if the stage is moveable. If the body tube of the microscope is moveable instead of the stage, move it downward to approach the specimen. As the specimen and the lens become closer, a spot of light should appear on the surface of the specimen. You should still be looking directly at the specimen and not through the microscope. Reposition the specimen, if necessary, so that the light falls on its center. Continue to close the gap between the specimen and the lens. Watch this gap while it decreases. When the specimen almost, but not quite, touches the lens, look through the eyepiece and slowly turn the coarse adjustment wheel in the opposite direction to increase the gap. You should notice that the illumination of the circular field of view will become brighter. Keep moving the coarse adjustment until the field of view reaches the maximum illumination. At that time the microstructure should come into focus although you may see it flash into view only momentarily as you pass it by. You have focused “going away” from the lens in order to avoid the damaging the objective lens by the specimen into it which you might have done had you focused going toward the lens. Now move your hand to the fine adjustment knob and turn it in the direction, which brightens the view until the structure comes into sharpest focus.

Study the structure. Using the mechanical stage movements, explore the etched surface, adjusting the focus, if necessary, as you go practice using both the stage control knobs simultaneously to move the specimen in various directions. Locate a good, clear area to sketch. If your specimen does not show a good, clear structure, ask the instructor to advice you whether you should repolish and re-etch.

Sketch the structure in the circle provided and fill in the information indicated.

Ask the instructor to check your sketch and your specimen while it is still on the stage.

Turn off the microscope light. Cover the microscope with dust cover. It is recommended that the microscope be left assembled at the close of the laboratory session because unnecessary assembly and disassembly will cause looseness to develop in the disturbed connections. Store the specimen properly according to the practice in the laboratory. Be sure it is identified with an index number and that a record of it is kept with it or in an organized filing place in the laboratory.