Resolution is the degree of detail that is retained in the magnified image. Oil immersion lenses has short focal length ie the plane of focus is near the lens and therefore there is a short working distance between the objective lens and the specimen. Short working distance results in a very shallow depth of field , ie only a very thin section of the specimen cen be in focuss at any one time. Oil immersion lens also have an RI in common with glass rather than air which improves resolving power. Eucaryotes have enought contrasting structures and can be viewed without staining ie live cells observed.
Dyes are organic compounds that have affinity for specific cellular material and increase contrast.
A smear is prepared air dried, heat fixed and flooded with the dye for a minute, washed, blotted, dried and observed using an oil immersion lens. Negatively charged dyes are repelled by the negative charges on the cell wall of bacteria and hence the background is colored but not the cell. Used to demonstrate capsulated microbes. A fluorescent substance emits light of one color when light of a different color shines on it. Used in medical microbiology and immunology laboratories.