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.
Wow, thanks! So how close is actually helpful? I found a list of household oils and refractive indices and mineral oil is around 1.47-1.48 -- would that help almost identically or would it be not a lot of help.
I do not know, but anything would be an improvement; just apply the oil (a bubble connecting slide through air to lens surface) and check the detail of objects in sample I assume. Arbitrarily anything above 1.4 would be pretty good. Amazing you found a list; surprising alcohol so high ?
Mineral oil should work. I sometimes use on a professional immersion objectives a mixture of drugstore-bought liquid paraffin (sold as a laxative) and sewing machine oil. Both are just highly purified mineral oil. Liquid paraffin is thicker than sewing machine oil, mixing them in 1:1 ratio gives just the right viscosity.
The downside of using this mixture instead of a commercial immersion oil is that it's harder to clean. Cleaning objectives after each use is required in my lab, and getting rid of paraffin using microfibre cloth soaked in a bit of ethanol and diethyl ether is tedious as opposed to cleaning commercial immersion oils.