Niddah, Chapter 2, Mishnah 4. This mishnah continues a discussion begun in the previous one, which spoke about the bedikah, the internal examination, that a woman carries out after marital relations. There the halachah turned on when the examination took place. Was it done right away? Then blood on the cloth is assumed to have already been present during the relations, with all the stringencies that follow from that. Was it done le'achar zman, once some amount of time had gone by? Then the tumah of husband and wife rests on doubt alone, misafek, and no chatas obligation falls upon either of them, because we have no way of establishing that the blood was there while they were together.
Naturally this leaves us wanting a definition, and that is exactly where our mishnah begins: "Eizehu achar zman?" What length of delay turns a bedikah into a late one?
The measure the mishnah supplies is not read off a clock but drawn from everyday behavior: "Kedei sheteired min hamitah vesadiach paneha" - as long as it takes her to get off the bed and to rinse her face. The word "face" is used here as a refined manner of speech; the intent is that she washes herself. Once such an interval has gone by, we no longer call the examination immediate. Blood on the cloth may have come out during those very moments, after the relations had finished, and not before. Since neither possibility can be ruled out, the tumah attaching to the two of them is only a tumah of doubt, and there is no chatas.
Next the mishnah turns to a longer delay with the words "ve'achar kach": what if the bedikah came still later than that? Should she then find blood, she becomes tmei'ah me'eis le'eis, retroactively for the previous twenty four hours, following the principles of retroactive tumah set out earlier in this perek. Nevertheless, "einah mitamah es bo'alah": her husband is not drawn into that tumah together with her. And it goes without saying that neither of them owes a chatas.
Rabbi Akiva takes issue with this last ruling. "Rabbi Akiva omer: af mitamah es bo'alah" - in his view the retroactive tumah extends to the husband too, and he becomes tamei along with her. The Gemara points out, though, that we do not pasken this way. The halachah follows the tanna kamma, the opening opinion of the mishnah: the twenty four hour retroactive tumah touches her and her alone.
The mishnah ends by identifying common ground between the two positions: "Umodim chachamim leRabbi Akiva bero'ah kessem, shemitamah es bo'alah." Where a woman finds a kessem, a bloodstain on a cloth, the chachamim grant Rabbi Akiva's point. A stain of that sort forbids her to her husband, and if they were together after she had seen it, the tumah does not remain hers alone; he becomes tamei as well.
What emerges from this mishnah is how carefully the halachah measures elapsed time. The brief moments of getting up from the bed and washing are enough to move a case out of the realm of certainty and into the realm of doubt, and a still longer wait moves it further, confining the retroactive tumah to the woman while leaving her husband outside it.