Oil immersion and focusing

The following are the measures to be kept in mind when adding immersion oil to the slide:

• Before adding immersion oil to the slide, ensure that the smear is facing upwards (Figure 1).

Figure 1: Slide should Face Upwards when Mounted on the Microscope Stage

• Add a drop of immersion oil, using the applicator (dropper bottle).
• The drop must fall freely onto the smear, so that the oil applicator does not touch the slide and get contaminated (Figure 2).

Figure 2: Oil Dropped onto Slide with an Applicator

• Mount the slide on the microscope and use the 10X objective lens to focus the smear, scan and look for mucoid material.
• Carefully rotate the 100X objective lens over the slide, adjust, sharp focus and observe under immersion oil.

Do’s and Don’ts:

• The 100X objective is the only lens requiring immersion oil.
• Keep the immersion oil away from other lenses.
• Use a good quality immersion oil.
• The immersion oil must have a medium viscosity and a refractive index (RI) greater than 1.5.
• Never allow the oil applicator to touch the slide.
• Do not use cedar wood oil diluted with xylene as it leaves a sticky residue on the lens and destroys the lens over time. 

Resources

• Guidelines for Laboratory Consumables, Central TB Division, 2019.
• Laboratory Diagnosis of Tuberculosis by Sputum Microscopy, GLI Initiative.

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