Mishnayos Yadayim Perek 1 Mishnah 1

מי רביעית - water that has in it the measurement of one-quarter of a LOG, which is an egg-and-a-half [in volume].

נותנים לידים לאחד אף לשנים – for one [person] we require ab initio one-quarter of a LOG. Even for two [people] if there was in it from the outset a quarter of a LOG and the first [person] poured [water over his hands] and afterwards the second [person], even though there wasn’t a quarter [of a LOG] for the second [person], it is all right, since they come from the remnants of purity (see Talmud Hullin 107a). But Maimonides [brings] an explanation, that specifically it is water for the second pouring [i.e., pouring water on the hands prior to consuming Terumah/heave offering (for a Kohen)] that we don’t we require a quarter of a LOG for each person, since his hands had already been purified from the first rinsing, but the first pouring, there cannot be less than one-quarter of a LOG for each person. But my Masters/Teachers did not explain it thusly.

מחצית לוג לשלשה או לארבעה – according to the measurement of one-quarter of a LOG for two [people], it would be enough to have one-quarter of LOG plus a half [of that amount] for three [people], but since that are a greater number of people, we are concerned lest they are not exacting well to pour [water only] up to the joint , for each person defines exactly regarding [how much he uses] because of his fellow. However, if there is one-quarter of a LOG and a half an the first [person] from them poured one-half of one-quarter of a LOG, the remaining one-quarter of a LOG will be sufficient for the two others, for even at the outset, it is enough for two [people] with one-quarter [of a LOG], all the more so now that they come from to leave over/preserve ritual purity. But if it was not done/take effect from this, these words [apply] were there didn’t remain one-quarter [of a LOG of water] for the last two [people].

ובלבד שלא יפחות לאחרון שבהן מרביעית – that there will be in the vessel one-quarter of a LOG at the time when the last person pours water on his hands/ritually washes them. But the Halakha is not according to Rabbi Yossi.

מוסיפין על השניים ואין מוסיפין על הראשונים – the law of pouring water on/washing the hands that requires washing the hands until the joint, and after he washed his hands with one pouring even though his hands are pure if he wiped them with a napkin, nevertheless, if he goes back and touches the water that is on the napkin, his hands are impure, and if a loaf of Terumah/heave-offering came in contact with them, it is impure, and therefore, it is the practice that he does another pouring of water to purify the first waters that are upon his hands, but now it is stated that if he placed water a second time but they didn’t reach the joint, he is able to add upon them and to pour water from a place that he didn’t pour there initially, and the first waters that are upon his hands are purified. But on the first waters, if they didn’t reach the joint, he is not able to add, but rather he must dry his hands and return and pour water at once that they should reach the joint.