Commentaire sur Nida 6:12
שְׁתֵּי שְׂעָרוֹת הָאֲמוּרוֹת בַּפָּרָה וּבַנְּגָעִים, וְהָאֲמוּרוֹת בְּכָל מָקוֹם, כְּדֵי לָכֹף רֹאשָׁן לְעִקָּרָן, דִּבְרֵי רַבִּי יִשְׁמָעֵאל. רַבִּי אֶלְעָזָר אוֹמֵר, כְּדֵי לִקְרֹץ בַּצִּפֹּרֶן. רַבִּי עֲקִיבָא אוֹמֵר, כְּדֵי שֶׁיְּהוּ נִטָּלוֹת בְּזוּג:
Les deux cheveux dont il est question concernant une génisse rouge [c'est-à-dire que deux cheveux blancs ou noirs l'invalident, voir Parah 2: 5], et [ceux dont il est question] concernant les nega'im [taches malades sur la peau, les vêtements ou les maisons qui créent impureté; c'est-à-dire que deux poils sur une tache sont un signe d'impureté, voir Nega'im 4: 4], et en ce qui concerne ceux dont il est question n'importe où, [les poils doivent être assez longs] pour plier leurs pointes pour [toucher] leurs racines, selon Rabbi Yishmael. Le rabbin Eliezer dit: [assez long] pour être pincé par un ongle. Rabbi Akiva dit: [assez long] pour être enlevé avec des ciseaux.
Bartenura on Mishnah Niddah
English Explanation of Mishnah Niddah
Rabbi Eliezer says: long enough to be grasped by a finger-nail.
Rabbi Akiva says: long enough to be taken off with scissors.
Today's mishnah clarifies how long a hair must be for it to count as a hair.
Section one: Besides the context of our chapter hairs that are a sign of human puberty there are several other "halakhic hairs." The first is that black or white hairs can disqualify a red heifer from being used in the purification ritual (see Parah 2:5). The second is that two hairs are a sign of impurity in a leprous spot (see Negaim 4:4, 10:2-3). The issue of hair also comes up in the commandment that a nazirite shave all of his hairs at the end of his naziriteship, and that a metzora (a person with skin disease) shave when becoming pure. If either of them leaves two hairs, the shaving doesn't count.
In all of these cases, Rabbi Ishmael holds that the hair must be long enough that one could take the tip and bend it back to the root.
Rabbi Eliezer says that it need only be possible to grasp the nail with one's finger nail. Rabbi Akiva says that it must be long enough that one could cut it with scissors. According to commentators, this is the smallest measure.