We've tried regular canola oil, Wesson canola oil, and Wesson vegetable oil. In all three, the handset can still be seen. The Pyrex borosilicate glass should visibly disappear. These tubes are still clearly visible, so the diffraction of light is not the same as with oil. The tubes themselves could be useful for something else, the ends are plain (flamed I think) and have a silver sticker that says 'Made in China'.
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