Negaim, Chapter 1, Mishnah 6. The previous mishnah taught Rabi Akiva's ruling that when a day of examination falls on Shabbos the kohen's visit is postponed until after Shabbos, and it closed by noting that this delay can work out either as a leniency or as a stringency. Our mishnah opens with the first half of that statement: "Keitzad lehakeil," in what way can the delay be a leniency? It then lists case after case in which, had the kohen looked on Shabbos, the person would have been declared tamei, but because the kohen came only on Sunday and something had changed in the interim, he goes out tahor.
What the mishnah takes for granted
The mishnah assumes that the reader has learned the parshiyos in Chumash and knows the simanei tumah of a nega: a black hair inside the nega that turned white (se'ar lavan), a patch of healthy flesh inside the nega (michyah), and the spreading of the nega (pisyon). Each of these, when found by the kohen, can make the person a full metzora.
It further presumes that the reader recognizes a group of skin conditions which are not tzara'as themselves, yet block the signs of tzara'as from taking effect. A shechin is a place where the skin was ruined by any agent other than fire, whether through injury or through sickness. A michvah is that very same ruined skin, except that here the cause was fire, or an object that fire had heated. Michyas hashechin and michyas hamichvah refer to skin that has since healed within such a sore or such a burn. Finally there is bohek, a term appearing here for the first time: a white fainter than even the faintest of the four shades listed earlier, fainter than krum beitzah. It is nothing more than a coloration of the skin, and carries no status of a nega at all.
Cases of white hair
"Haya bo se'ar lavan," when Shabbos came the nega held a hair that had turned white, and a kohen examining at that moment would have pronounced him muchlat immediately. But "vehalach lo" the hair was no longer there by Sunday: perhaps it fell out, perhaps its color reverted to black. Either way, he emerges tahor.
"Hayu levanos vehishchiru," there were two white hairs on Shabbos and by Sunday they had turned black again. "O achas levanah ve'achas shechorah vehishchiru sheneihen," or one was white and one was black, and by Sunday both were black.
"Aruchos vehiktziru," the hairs had reached the required length and then shrank back. There is indeed a measure for how long such a hair must be. One approach: it must be long enough to be folded down until its tip touches the root from which it grows. The Rambam measures it by whether a shears could take hold of it and trim it, for a hair too stubby leaves the blades nothing to grip. Accordingly, hairs that satisfied the measure while it was still Shabbos, but for whatever reason fell short of it by Sunday, no longer constitute a siman. "Achas arukah ve'achas ketzarah vehiktziru sheneihen," or the pair consisted of one hair of sufficient length and one too short, and by Sunday both fell below the measure.
These white hairs have to be located within the white of the nega itself. Once a shechin pushes its way in, leaving the hairs no longer ringed entirely by the original tzara'as, no tumah results. That yields the mishnah's cases: "o chilkan," the shechin cut through the nega, so that what still surrounds the hairs amounts to less than a gris; "nismach hashechin lishteihen o le'achas mehen," the shechin came up against both hairs, or against just one; "hikif hashechin lishteihen o le'achas mehen," the shechin formed a ring around the two, or around a single one.
"O michyas hashechin vehamichvah umichyas hamichvah vehabohek." All of these do the same work as the shechin: the healed skin within a shechin, a michvah, the healed skin within a michvah, and a bohek, each of them interrupts between the hairs and the whiteness of the se'eis or baheres and their toldos. And in every one of these cases, the change occurred on Sunday, when the kohen finally came.
Cases of michyah
A hair inside the nega that was once dark and turned white, provided the white of the nega is of the requisite shade, functions on its own as a siman tumah. The michyah operates the same way: a spot of sound flesh within the nega is in itself sufficient to render the person a full metzora at once.
"Haysah bo michyah vehalchah lah." A michyah was present while it was still Shabbos, and by the time Sunday arrived it had disappeared.
The michyah has its own halachos. It must be square, and it must sit in the middle of the nega, surrounded by it, and it must be the size of an adashah, a lentil. Hence the mishnah's list: "Haysah meruba'as venes'as agulah o arukah," it was square and became round or elongated; it was in the middle and moved off to the side; "mechuneses venispazrah," it was gathered together as one unit and became scattered, so that even though it is still inside the nega it is now a set of specks rather than one continuous patch.
"Uva hashechin venichnas lesochah, hikifah, chilkah o mi'atah." The shechin, which interrupts between the white hairs and the nega, likewise interrupts by the michyah: it entered into the michyah, or encircled it, or divided it, or reduced it below the size of an adashah. And here too the same is true of michyas hashechin, of the michvah, of michyas hamichvah, and of the bohek, that shade of white which is not one of the four and is dimmer even than krum beitzah.
In all these cases, had the change occurred by Sunday, the person is tahor, because the michyah is no longer a proper michyah sitting whole in the middle of the nega, surrounded by the shades of white listed at the beginning of the chapter or their reddish mixtures.
Cases of pisyon
"Haya bo pisyon, halach lo hapisyon." Say this was the examination at the end of the second week. On Shabbos the nega had already spread, and a kohen looking then would have declared him a definite metzora. By Sunday the spread was gone.
"O shehalchah lah ha'om," or the om, the main body of the nega, disappeared, leaving only the area into which it had spread. "O shenisma'atah, ve'ein bazeh uvazeh kegris," or it shrank, so that neither the body nor the spread contains the size of a gris.
The mishnah then lists five things: "hashechin umichyas hashechin vehamichvah umichyas hamichvah vehabohek," a sore, the healed skin inside a sore, a burn, the healed skin inside a burn, and a bohek. Not one of them qualifies as a nega, yet all of them, in the mishnah's words, are "cholkin bein ha'om lapisyon": they separate the original body of the nega from the area to which it spread. Thus even if the spread was still attached to the main patch when Shabbos began, should one of these have wedged itself in between by Sunday, the man has come out ahead: the postponement served him as a leniency, and he is tahor.
"Harei eilu lehakeil." These, then, are the cases of leniency. In each one, an examination on Shabbos would have meant tumah, but because the kohen came only after Shabbos, and in the interim the nega was diminished, or broken up, or otherwise altered, what he sees is no longer a nega that makes the person tamei.