Midrash su Levitico 13:4

1) (Vayikra 13:4) ("And if it is a white bahereth in the skin of his flesh, and its appearance is not deeper than the skin, and its hair has not turned white, then the Cohein shall quarantine the plague-spot for seven days.") This tells me only of a white bahereth. Whence do we derive for inclusion a (white) se'eth? From (Vayikra 13:10) "a white se'eth." Whence are other (white) appearances derived? From "And if a bahereth."

2) I might think that just as it (bahereth) is third in order of the verse (Vayikra 13:2), it is third in order of appearance (i.e., of brightness); it is, therefore, written "white." It is white and there is no (white) above it in brightness. And what is its whiteness like? Like (the whiteness of) snow, as it is written (Bamidbar 12:10) "And, behold, Miriam was as leprous as snow."

3) I might think that every appearance of snow is tamei and that all other appearances are tahor. It is, therefore, written (Vayikra 13:39) "It is a bohak (brightness) … it is clean": It is clean, but everything above it (in brightness) is tamei.

4) From here they ruled: The appearances of negaim are two, which are four: Bahereth is bright as snow. Second to it (in brightness) is the plaster of the Temple (walls). Se'eth is (bright) as the membrane of an egg. Second to it is (the brightness of) white wool. These are the words of R. Meir. The sages say: Se'eth is (bright) as wool. Second to it is the membrane of an egg.

5) The variation of snow (in a nega) is like wine mixed with snow. The variation of plaster is like blood mixed with milk. These are the words of R. Yishmael. R. Akiva says: "adamdam" (reddish) in both (viz. Vayikra 13:24) is like wine mixed with water; but that of snow is more intense, and that of plaster, duller.

6) R. Chanina, the adjutant high-priest, says: The appearances of plague-spots are sixteen. R. Dossa b. Hyrcanus says: They are thirty-six. Akavya b. Mehalalel says: They are seventy-two. R. Yossi said: R. Yehoshua b. R. Akiva asked R. Akiva: Why did they say "The appearances of negaim are two, which are four"? R. Akiva: If not, what should they say? R. Yehoshua: They should say: From the membrane of an egg and above it (in brightness) is tamei. R. Akiva: To teach us that they combine with each other (to form the prescribed size for tumah). R. Yehoshua: Then let them say: Everything from the membrane of an egg and above (in brightness), and they combine with each other. R. Akiva: To teach us that if one is not expert in them and in their names he should not inspect negaim.

7) "and its hair has not turned white": its hair and not the hair of its michyah (a kind of scar, viz. 13:24). How so? If the bahereth were the size of a garis, and in it a michyah the size of a lentil, and a white hair in the midst of the michyah — if the michyah leaves, it is tamei by reason of white hair; if the white hair leaves, it is tamei by reason of michyah. R. Shimon rules it tahor because it is not the bahereth that turned the hair white, (but the michyah). They said to him: Is it not written (3) "if hair in the plague-spot has turned white"? It is a plague-spot, in any event.

7) (Vayikra 13:55) ("And the Cohein shall see, after the plague-spot has been washed, and, behold, the plague-spot has not changed its appearance, and the plague-spot has not spread, it is unclean. In fire shall you burn it; it is p'cheteth in its karachath or in its gabachath.") "after the plague-spot has been washed, and, behold, the plague-spot has not changed its appearance and the plague-spot has not spread, it is unclean.": If it has not changed and not spread, it is tamei; but if it changed and did not spread, it should be examined as in the beginning. These are the words of R. Yehudah. And the sages say: It is tamei by reason of omed (viz. Vayikra 13:4 above). How, then, am I to understand "and, behold, it has not changed"? From any appearance that renders it subject to tumah (even if it changed from green to red).

8) "and its hair has not turned white": and not the hair of part of it. How so? A bahereth and its michyah is a garis, and there is white hair in the midst of the bahereth — if the michyah leaves, it is tamei by reason of white hair; if the white hair leaves, it is tamei by reason of michyah. R. Shimon rules it tahor because it is not a bahereth the size of a garis that turned it white, (its michyah having rendered it less than a garis). And all agree that if there is in the site of the white hair the size of a garis (aside from the michyah), it is tamei.

9) "and its hair has not turned white, then he shall quarantine" — the black hair does not detract (from the required tumah size of the bahereth, for which reason he must be quarantined). The disciples asked R. Yossi: If there were a bahereth with black hair in it, do we suspect that it detracts from its (required tumah) size of a garis? He answered: If there were a bahereth with white hair in it do we suspect that it detracts from its size of a garis! (Why, then, should we suspect so for black hair?) The disciples: If they said so for white hair, which is a sign of tumah, should we say so for black hair which is not a sign of tumah? R. Yossi: In an instance of ten white hairs, where only two are signs of tumah, do we suspect that the remaining ones detract from the bahereth's (required tumah) size of a garis? The disciples: No, but that may be so with white hair, which is of the kind that becomes tamei. Would you say the same for black hair, which is not of the kind that becomes tamei? R. Yossi: Even black hair turns into the kind which becomes tamei, and, (if this does not suffice for you), it is written "and its hair has not turned white, then he shall quarantine" — the black hair does not detract (from the required tumah size of a garis). "then the Cohein shall quarantine the plague-spot for seven days" — to begin with.