Replying to myself, I found 1899 paper which is more explicit on the matter, and shows how Rayleigh was not all that certain about the results: The compari

Replying to myself, I found 1899 paper which is more explicit on the matter, and shows how Rayleigh was not all that certain about the results: The comparison of the present with former results throws an interesting light upon molecular magnitudes. It has been shown (Proc. Roy. Soc. March 1890) that the thickness of the film of olive-oil calculated as if continuous, which corresponds to the camphor-point, is about 2.0 μμ while from the present curves it follows that the point at which the tension begins to fall is about half as much, or 1.0 μμ [...] If we accept this view as substantially true, we conclude that the first drop in tension corresponds to a com- plete layer one molecule thick, and that the diameter of a molecule of oil is about 1.0 μμ XXXVI. Investigations in Capillarity:—The size of drops.—The liberation of gas from supersaturated solutions.—Colliding jets.—The tension of contaminated water-surfaces If we assume that the "about 2.0 μμ" value is just the previously mentioned 1.63 nm value rounded up, then that throws a wrench into the story, in particular this bit from blog post > Rayleigh’s final result was 1.63 nanometers. Olive oil is mainly composed of fat molecules called triacylglycerols, and we now know that they measure about 1.67 nanometers in length, implying that Rayleigh’s “primitive” estimates were off by just 2 percent is more of a numerological coincidence, the actual estimate that Rayleigh gives is half of that!