Been using this Rayleigh water tester last evening and it works ok, the magnifying lens is 90 mm and 11 3/4" f.l. I put it that far from the light ( white compact flourescent ) there's a 1" opening for the light and white paper to diffuse it. Using the flashlight to level the flat. Once I lined up the reflections and looked in the splitter window and was lucky to see fringes. Now the round flat looks like 5 fringes concave, not 20 from the no lens test. I tilted the lens to move the 2 reflections of the bulb off the flat. I think plain water works ok, didn't try an aluminumized flat and food color yet. They are glued to the spiders. An interesting tester. Thanks for info.