OPINIONS: The Mishnah says that Jewish women may go out on Shabbos while "Re'ulos." What does "Re'ulos" mean?
(a) RASHI here explains that it refers to a woman whose entire head is completely covered, except for her two eyes. This is what the word means in Yeshayah (3:19).
(b) Rashi in Yeshayah, however, explains that the women used to cover their entire head except for one eye (such attire is considered provocative, because it entices men to want to see more of her face. This is why it is called "Re'ulos," from "Ra'al" -- poison).
(c) Rashi in Yeshayah gives another explanation. "Re'ulos" refers to thick coats that women wrapped around themselves.
(d) The RAMBAM here writes that "Re'ulos" refers to bells that hang from their garments.
2) HALACHAH: IMMERSION IN A RIVER
Shmuel states that a river's waters increase from the river's own subterranean sources, and therefore one may immerse in a river throughout the entire year.
TOSFOS (DH d'Amar Shmuel, and in Bechoros 55b, DH Ein) cites RABEINU TAM who rules in accordance with Shmuel.
Rabeinu Tam in Bechoros adds that even those who prohibit Tevilah in rivers during certain seasons (based on the rulings of Shmuel's father and the other Amora'im) agree with Shmuel's ruling in several cases:
(a) All the rivers in Eretz Yisrael are fit for Tevilah. Since Eretz Yisrael is higher than all other lands (Zevachim 54b), the land sheds its rainwater and its rivers flow from subterranean sources alone. (See Mikva'os 8:1, "Eretz Yisrael is Tahor and its Mikva'os are Tahor," -M. KORNFELD.)
(b) The same may apply to all other rivers on mountains.
(c) Rabeinu Tam adds that all Amora'im agree that according to Torah law, one may immerse in a river even during the rainy season. It was the Rabanan who prohibited Tevilah in a river due to Mar'is Ayin (that is, it looks as if the person is immersing in rainwater). He offers two logical grounds for this assertion. First, rainwater trickles into the river drop by drop and is therefore Batel (annulled) by the river water. Second, all rivers join with the sea at some point (Koheles 1:4), and since the sea is valid for Tevilah, so are the rivers.
HALACHAH: The SHULCHAN ARUCH (YD 201:2) rules that if there is more rainwater then river water in a river, one may immerse only if the water is kept stationary. Although the REMA recommends following this ruling, he adds that there is an opinion (that of Rabeinu Tam) that one may immerse in a river with more rainwater than river water. The REMA concludes that the custom is to immerse in rivers throughout the entire year in places where there are no Mikva'os.
3) A COIN AFFIXED TO ONE'S FOOT
QUESTION: The Mishnah states that a woman may go into Reshus ha'Rabim with a stone, nut, or coin wrapped in her shawl serving as a button. The Gemara explains that all three are permitted when wrapped in the shawl prior to Shabbos. She is forbidden, though, to place a coin in her garment on Shabbos itself, because a coin is Muktzah. She may place a stone in her garment, as long as she planned to use it on Friday, prior to Shabbos (RASHI).
Why is the stone different from a coin? If she planned before Shabbos to use the coin on Shabbos, then it should also be permitted!
(a) The RITVA and TUR (OC 303) explain that planning on Friday to use a coin on Shabbos does not work, because most people do not use a coin as a button, and thus her desire to do so is thus negated ("Batlah Da'atah"). If she actually uses the coin as a button before Shabbos, then its usage is redefined as that of a button and it is not Muktzah on Shabbos. In order to use a stone on Shabbos as a button, on the other hand, it suffices to plan to use it, because a stone is commonly used as a button.
(b) The SHILTEI GIBORIM cites the SEMAG who says that when a woman wraps a coin in her shawl as a button, it appears like she is just trying to carry the coin into Reshus ha'Rabim in a permitted manner, by circumventing the prohibition of Hotza'ah, because it is very uncommon to wear a coin as a button. However, when she uses the coin as a button before Shabbos, it becomes designated as a button and she may go out with it on Shabbos.