Note 13 — PART III. Scientific Pursuits: (continued 3)
Reading edition
The thinnest substance ever observed is the aqueous film of the soap-bubble previous to bursting; yet it is capable of reflecting a faint image of a candle, or the sun. Hence its thickness must correspond with what Sir Isaac Newton calls the beginning of black, which appears in water at the thickness of the seven hundred and fifty thousandth part of an inch.