He steeped pepper in water and left it sitting for weeks, because he wanted to know why pepper bites the tongue. What he found was swimming. Then he went round the rest of the house the way you'd walk a transect — rainwater standing in a blue pot in the yard, well water, water from the sea, melted snow — noting when he'd collected each one and what the weather had been doing beforehand.
So it's a field notebook, really. A draper in Delft, 1676, lenses he'd ground himself, writing to London with counts of animals nobody else had laid eyes on. They didn't take it on trust; Hooke had to sit down and repeat the whole thing before anyone in the Society would sign off. Fair enough. Only one person had been to that habitat.
He wrote in Dutch, in plain shopkeeper's prose, no Latin at all — so somebody at the Royal Society had to carry him over into English before anyone could argue with him. The word for the little swimmers in his letters is diertgens, small beasts; animalcule is the translator's word, not his.
The lens was a bead of glass smaller than a lentil, Kit, and he ground it himself with no training. Roughly 200 to 270 times, when the best compound microscopes of the day managed maybe 50. No wonder Hooke needed a few goes before he could see it too.