(Rav Shmuel bar Rav Yitzchak): A woman should not immerse in a port:

(Rashi - the ships loosen the mud.) Even if she will not find anything when she comes out, perhaps there was mud on her feet, and it came off while she was leaving. (Tosfos deletes this from the text. He explains that she fears being seen, and will not immerse properly.)

Shmuel's father made Mikva'os for his daughters in springtime, and put mats (in the river) for them in fall. (Rashi - Notfim (rainwater) is Metaher only if it is gathered, but not if it is flowing. Only Zochlim (water that comes from the ground, i.e. a spring) is Metaher when flowing. He made Mikva'os in springtime, lest the river is mostly Notfim. In fall, he put mats on the river floor lest mud be a Chatzitzah in their feet. R. Chananel - the mats were for privacy.)

(Rav Gidal): If a woman fed a cooked food to her son before immersing (and did not check herself in between), her immersion is invalid;

Even if she does not see a Chatzitzah now, perhaps there was one, and it came off while she was leaving.

(Rami bar Chama): A scab from bloodletting is not a Chatzitzah for three days. (After bloodletting, it is still soft. It is like the skin);

After three days (it is hard), it is a Chatzitzah.

(Mar Ukva): Moist Lifluf (mucus of the eye) in the eye is not a Chatzitzah. If it is dry, it is a Chatzitzah;

It is considered dry when it starts to turn yellow.

(Shmuel): Mascara in the eye is not a Chatzitzah. On the eye (lid), it is a Chatzitzah;

If the eye twitches (this thins out the mascara, it becomes insubstantial), even on the eye it is not a Chatzitzah;

(R. Yochanan): If she opened her eyes too much or closed them too tightly, the immersion was invalid. (This is a Chatzitzah above or below the eye, respectively.)

(Reish Lakish): A woman should immerse in her normal posture, like the way we inspect Tzara'as:

(Mishnah): When inspecting Tzara'as (in the underarm or near the Ervah), a man poses as if he is picking olives or hoeing (if the Nega cannot be seen, he is Tahor);

A woman poses as if she is weaving or nursing.

(Rabah bar bar Chanah): If one hair is knotted (by itself), it is a Chatzitzah. Three hairs knotted together are not a Chatzitzah. I do not know about two.

(R. Yochanan): Only one is a Chatzitzah (Two are not.)

(R. Yitzchak): Mid'Oraisa, if there is a Chatzitzah on the majority (of the body; Rashi - of the hair), and the person is Makpid (wants to remove it), this invalidates the immersion;

Chachamim invalidate the immersion if there is a Chatzitzah on the majority and he is not Makpid, or a Chatzitzah on the minority that he is Makpid about. They decreed due to a majority that he is Makpid about.

Question: Why didn't Chachamim decree about a Chatzitzah on the minority that he is not Makpid about, due to a minority that he is Makpid about?

Answer: A minority he is Makpid about is only a decree. We do not decree to guard a decree!

(Rav): When a Nidah immerses in the proper time (after the seven days), she must immerse at night (of the eighth -- before this, immersion is not Metaher). If she immerses afterwards (day eight or later), she may immerse by day or at night.

(R. Yochanan): In any case, she must immerse at night, lest her daughter (think that her mother used to immerse on day seven during the day and) do similarly.

Rav retracted to hold like R. Yochanan;

(R. Chiya bar Ashi citing Rav): A Nidah must immerse at night, in the proper (first possible) time or afterwards, lest her daughter follow her example.

In Narash (a place), Rav Idi enacted that women immerse on the eighth day, due to (danger at night due to) lions;

Rav Acha bar Yakov, Rav Yehudah and Rava enacted this in their respective cities due to thieves, the cold (lest women get sick, or immerse hurriedly) and Nochri gatekeepers. (Alternatively, they would close the gates of the city at night, or there was danger of falling on the way to the Mikveh.)

Question (Rav Papa): Nowadays, women are stringent to consider themselves Safek Zavos (like R. Zeira taught). They should be allowed to immerse on the seventh (clean) day! (Only a Nidah must wait until night, and the seven days of Nidah already passed.)

Answer (Abaye and Rava): They may not, due to R. Shimon's teaching:

(Beraisa): "V'Achar Tithar" -- after all the (seven clean) days. Tum'ah may not interrupt in the middle;

R. Shimon expounds "v'Achar Tithar" -- after an action, she is Tehorah. (She may immerse even after one moment of (morning of) day seven). However, Chachamim forbid doing so, lest she come to a Safek. (Tosfos - R. Shimon says that Chachamim forbade immersing on day seven, lest she have Bi'ah, which is a Safek. Ramban - R. Shimon permits Tevilah, but says that Chachamim forbade Bi'ah that day.)

(Rav Huna): A woman may do Chafifah (wash herself or her hair) on Sunday and immerse on Tuesday, just like one may immerse on Motza'ei Shabbos, the Chafifah was two (Halachic) days earlier on Erev Shabbos;

She may (do Chafifah on Sunday and) immerse on Wednesday, just like one may immerse on Motza'ei Yom Tov when Yom Tov follows Shabbos, three days after Chafifah (on Erev Shabbos);

She may immerse on Thursday, just like one may immerse on Motza'ei Yom Sheni of Rosh Hashanah when Rosh Hashanah follows Shabbos, four days after Chafifah.

(Rav Chisda): When it is possible (to do Chafifah on the at night of Tevilah), she does so. When it is impossible, she cannot (then we rely on Chafifah two to four days before Tevilah). We do not say 'just like'. (R. Akiva Eiger - the Ran holds that this applies only to Rosh Hashanah, but Chafifah for the sake of immersion is permitted on the second day of (any other) Yom Tov observed in Chutz la'Aretz.)

(Rav Yemar): We say "just like," except for immersion four days after Chafifah on Motza'ei Yom Sheni of Rosh Hashanah when Rosh Hashanah follows Shabbos, for the Chafifah may be done that night before immersing. (A four day delay is too long. When Rosh Hashanah follows Shabbos, if the night of immersion is the first or second night of Rosh Hashanah, we must rely on Chafifah two or three days ago.)

(Mereimar): The Halachah follows (the stringencies of) Rav Chisda (we never say "just like") and Rav Yemar. (She may not immerse four days after Chafifah, even due to Shabbos and Rosh Hashanah.)

Question: May a woman do Chafifah at night and then immerse?

Answer #1 (Mar Zutra): It is forbidden. (She might rush, for she is anxious to be with her husband);

Answer #2 (Rav Chinena of Sura): It is permitted. (Maharsha switches these opinions; Bach does not, but he says that the following question is against Mar Zutra.)

Question (Rav Ada): A case occurred in which the Reish Galusa quarreled with his wife (on the last day before she was due to immerse). Rav Nachman bar Yitzchak went to appease her that night.

The wife: What is your rush? I will immerse tomorrow!

Rav Nachman understood that she thought that she cannot immerse tonight because she had not done Chafifah.

Rav Nachman: (You can do Chafifah now and immerse tonight, for) surely you do not lack slaves (to heat the water), basins, or a seat (to sit on at the time)!