"Everything in water shows bigger than outside it."
Leeuwenhoek was aware of the additional magnification that his lenses got from a glass tube:
The reason why I caused it to be drawn hanging in water is that the filaments hang better separated thus than without water, and so can better be distinguished, especially if one allows the light of a candle to shine through the glass. But knowing that everything in water shows bigger than outside it, we must not take the filaments to be as thick as they are drawn here.
This additional magnification may have been what Leeuwenhoek referred to as the method he did not tell anyone about. See the Related page under Learn more below.
He did not get very far with the observations of vine sap. The letter ended with a reference to one of his two gardens, this one within Delft's city walls.
After this, I went to my garden, which lies within this town, and there also observed the sap dripping from the vine but could not discover any living creatures in it, except only a little worm that was uncommonly big, compared with the other living little animals.
I thence went to my garden outside the town and there too observed the sap from divers vines, but could not discover any living creatures in it. I cut off two pieces from vine-shoots to make them drip the more and went to examine the sap again the next day, but could not find in it anything living. I took a new glass bottle and caught the sap in it, and carried it home and examined it, and again could not discern any living little animals in it.
I am now busy finding out if possible, why there are living little animals in one sap and none in the other.
It took Leeuwenhoek two months to answer his question. Letter 31 of June 14, 1680, described and illustrated the little animals that he found in the sap.