Negaim, Chapter 2, Mishnah 4. This chapter sets out the conditions that make a valid examination of a nega possible: the complexion of the person examined, the hour and the lighting, and here, the eyesight of the kohen himself and the illumination of a house being inspected.
A kohen whose vision is deficient
The mishnah's opening ruling reads: "Kohen hasumeh be'achas me'einav, o shekehah me'or einav, lo yir'eh es hanega'im." Should a kohen have lost sight in one eye, or should the light of his eyes have grown faint, he is unfit to inspect nega'im. The Tosfos Yom Tov distinguishes precisely between these two disqualifications. Faintness of sight bars him only if it has settled on both eyes; where one eye is weak but the other still sees fully, his examination stands. Actual blindness is stricter, and even in a single eye it renders him unfit.
"Shene'emar lechol mar'eh einei hakohen": Scripture makes the verdict depend on everything that the kohen's eyes are able to take in, and sight that has been weakened falls short of that completeness. These very words carry a second requirement as well, one taught in the mishnah just before ours: the daylight in which the inspection takes place must be at its full strength, which is why only particular hours qualify for viewing nega'im. Wholeness is demanded twice over, in the eye that looks and in the light that lets it look.
A dark house
"Bayis ha'afel, ein poskin bo chalonos": in a darkened house one does not cut windows. We are dealing here with nigei batim, where the Torah in Parshas Metzora describes the appearances of yerakrak and adamdam, a green or reddish tinge, showing on the wall of the house. Unlike a person or a garment, which are brought out into the daylight and are never examined indoors, a house cannot be moved: the kohen has no choice but to enter and look at the wall where it stands. Our mishnah teaches that even so, one does not improve the lighting for him.
The Rav explains this from the language of the Torah, "kenega nireh li," it appears to me like a nega: li velo leneri, to me and not to my lamp. The determination must rest on what the kohen's own eye perceives in the room's natural light, and not on illumination that he introduces from outside. From this the Chachamim said: in a dark house one does not open up windows for it.
The Tosfos Yom Tov cites a different understanding in the name of the Maharam. In his words, the meaning appears to be that if the windows are shut the kohen should not look at the nega until he opens them, and this is indeed the implication of the Tosefta, which states: a dark house whose windows are stopped up, one opens the windows and views its nega. What then does our mishnah mean when it says that one does not cut windows for it? According to the Maharam, our mishnah is speaking of a house that has no windows at all, and it teaches that one may not create new ones. But where windows exist and are merely closed at the moment, one opens them and examines the nega. The Rav, as we have seen, understands the mishnah differently and does not permit the window to be opened for the examination.
The mishnah thus closes the circle it opened: the kohen must bring whole eyes to the nega, and he must see it in the light that is naturally there, neither aided by his lamp nor, in the Rav's reading, by windows opened for the occasion.