Perhaps he was observing the layer making sure it had integrity? Oil layers famously have an optical effect (iridescence from interference of reflections). This

Perhaps he was observing the layer making sure it had integrity? Oil layers famously have an optical effect (iridescence from interference of reflections). This effect would transition as the layer goes from >= 1 molecule thick to <= 1 molecule thick (on average). It's likely possible to pinpoint this transition experimentally and then using the oil volume obtain the molecular layer thickness.