I've seen some microscopes use "immersion oil" -- like, an oil between the lens and the slide. What's this for and where can I find it? Do all microscopes need it? Can non-oil microscopes use oil or vice versa?
Glass apparently has a refractive index of ~ 1.5 while air is 1.0. This means that light velocity is slower in glass. When light from a sample slide passes through air to objective lens there is refraction or bending of light away from perpendicular. As a result, some of the light from the specimen is lost and does not enter the aperature of the lens. If you fill the air gap between slide and lens with oil with a similar refractive index as the glass slide (i.e. 1.5) then the light is no longer refracted away from lens. With this effective increased light entering the lens or increase in aperature; resolution of detail in the sample is increased. Yes you can skip it if you have adequate resolution to see the detail in specimen your are interested in.