Thank you, Rik!
So I have to add some more details!
The technique is very simple: take a glass jar full of warm (or hot) water, put one drop of essential oil, adjust lighting... and observe!
1. I've found that oils of conifers (pine, fir, juniper etc.) behave better. Although citrus oils are good too, you can just squeeze them out of their peels.
2. It's better to use light source which spectrum have distinct lines (e.g. fluorescent energy saving lamps), because it demonstrate prominent color banding even for thicker layers (where continuous smooth spectra show so-called "whites of high orders").
3. Colors are not so bright (comparing with soap bubbles and crossed polarizers), so you have to enhance them in image editor.
This is the scheme of my primitive setup:
