"The thickness of oil required to take the life out of the camphor movements lies between one and two millionths of a millimetre, and may be estimated with some precision at 1’6 micromillimetre."
Looks like a primary guess to me, even if the table lists more data points.
I definitely thought so too, in the first paper the 1.67 isn't even really his primary guess so definitely some presentist bias in the OP