Terumos, Chapter 8, Mishnah 7. Before us is another mishnah dealing with the law of giluy - liquids that were left uncovered - and this time regarding wine. The question under discussion is whether a strainer resting on top of a vessel is considered a covering, such that the liquid in the lower vessel remains permitted even though it has no actual covering over it.
The case:
The upper vessel - it holds the wine and serves as a meshameres (a strainer), and the wine drains down out of it.
The lower vessel - an ordinary vessel that receives the strained wine.
The question, then, is this: may one drink from the lower vessel, on the grounds that it is covered by the strainer above it?
In the words of the Mishnah: "Ha'meshameres shel yayin - asurah mishum giluy. Rabbi Nechemyah matir" - a wine strainer is forbidden because of giluy; Rabbi Nechemyah permits it.
According to the first Tanna, even the wine in the lower vessel is forbidden, since it is still considered uncovered: a snake may drink from the upper vessel, and its venom could pass through the strainer down into the lower vessel.
Rabbi Nechemyah, by contrast, permits drinking from the lower vessel, and holds that there is no concern of giluy here, because the venom does not pass through the strainer. Two explanations are offered for this:
The venom is thick and does not pass through the strainer.
According to the Bartenura's understanding - the venom floats on top, and for that reason it does not descend and pass through the strainer.
In summary: The halachah does not follow Rabbi Nechemyah, but rather the first Tanna. It emerges that wherever the law of giluy applies, a strainer is of no help in permitting the liquid in the lower vessel.