Through the microscope. Always begin your observations at low power. Focus on particles. Note that you cannot determine whether these are bacteria or debris.

Under oil immersion, you should be able to see the shape of the bacteria.

Low power

Use low power to center and focus the image. You will not be able to distinguish bacteria at this point. After you have focused the slide with low power, switch to the high-dry lens and focus using the fine adjustment. Then move on to the oil-immersion lens to see the bacteria.

Focus using the fine adjustment. The bacteria are more defined but individual cells cannot be seen. Move on to the oil-immersion lens to see the bacteria.

Oil-immersion objective

Place a drop of oil on the slide; the lens sits in the oil. Focus using the fine adjustment. Now you can determine the size, shape, arrangement, and gram reaction.