Negaim, Chapter 2, Mishnah 3. This chapter is occupied with the conditions under which a kohen may render a decision on a nega: the skin tone of the person being examined, the eyesight of the kohen, and, in our mishnah, the lighting available at the moment of the examination.
One clarification about the opening mishnah of the chapter before we take up ours. That mishnah spoke of a Germani, a person of an extraordinarily light complexion (the term is bound up with gerem, a bone, which is whitish in color), and of a Kushi, whose skin is very dark. Rebbi Yishmael responded there with a declaration of love for Klal Yisrael, offering himself as their atonement, and then said of them harei hen k'eshkeroa: the coloring of Yisrael is that of the boxwood, an intermediate Mediterranean shade. The Rav takes Rebbi Yishmael to be making the identical point as Rebbi Akiva: when the person under examination is a Germani or a Kushi, a dye of that middle shade is applied to the skin bordering the nega, so that the neighboring flesh should not skew the spot's appearance in either direction, and the comparison can be judged truthfully. The Tosfos Yom Tov, though, brings the Ravad in his peirush on the Sifra, who reads Rebbi Yishmael as disagreeing with Rebbi Akiva. Since complexions of this sort lie so far outside the ordinary range, people of such coloring drop out of the parshah entirely: no trustworthy reading can be taken from their skin as it is, and they are consequently not bound by se'eis, sapachas and baheres, nor by the four grades of white and the mixtures among them that were set out earlier. Such is the machlokes between the Rav and the Ravad in explaining Rebbi Yishmael.
The source: the kohen's full vision, and the day's full light
The Rav grounds our mishnah in the words lechol mar'eh einei hakohen, everything that the eyes of the kohen see. Chazal draw a comparison from here: a kohen whose eyesight is not whole, shechaser mar'eh einav, is unfit to inspect a nega; by the very same measure, a day, or one particular hour of a day, whose illumination is not whole, shechaser me'oro, is not a time for inspecting nega'im at all. Whole seeing on the part of the examiner demands whole light in the place where he stands.
The times and places that are disqualified
"Ein ro'in hanega'im bashacharis uvein ha'arbayim": we do not examine nega'im in the early morning or toward evening. As will emerge from the mishnah's own list, the morning here means the stretch before the third hour, and bein ha'arbayim means the time after the ninth hour. The hours are counted from dawn, and they are sha'os zemaniyos, relative hours: one divides the daylight into twelve equal parts, whether one measures from sunrise to sunset or, according to the shitah of the Mogen Avraham, from amud hashachar until tzeis hakochavim. These hours are accordingly longer in the summer and shorter in the winter.
"Velo besoch habayis": nor inside a house. This refers to nigei adam and nigei begadim, the nega'im of a person and of garments. Indoors one simply does not have the full brightness of daylight, all the more so in those days, when there was no electric lighting. We will learn later that with nigei batim, the nega'im of houses, there is no alternative: the kohen must enter and view the discoloration on the wall from within, and there is no way around it. But a person and a garment can be brought outdoors, and so they must be.
The mishnah continues, velo beyom hame'unan: an overcast day is likewise unfit, lefi shehakeheh nireh az, since under such a sky a weak white presents itself as a strong one. Where the surrounding light is muted, a pale shade gains from the contrast and reads deeper than it actually is.
"Velo batzohorayim": nor at midday, lefi she'az nireh keheh, because then the opposite distortion occurs. In blazing sunlight a genuinely bright white is washed out and appears dull, and the kohen would read it as weaker than it is.
Which hours remain
"Eimasai ro'in? Beshalosh, be'arba uvechamesh, uvesheva uvishmoneh uvetesha": the examination is conducted in the third, fourth and fifth hours of the day counted from dawn, and in the seventh, eighth and ninth. The sixth hour is left out, since that is tzohorayim, the height of the day. Anything before the third hour is left out, since that is shacharis, and anything after the ninth is left out, since that is bein ha'arbayim. "Divrei Rebbi Meir": this is Rebbi Meir's shitah, and it is also the position of the Tanna Kamma who opened the mishnah.
Rebbi Yehudah narrows the window further. "Rebbi Yehudah omer: be'arba uvechamesh, uvishmoneh uvetesha." He holds that the third hour is still included in the morning weakness of light, so the light is not yet adequate until the fourth hour, whereas Rebbi Meir holds that by the third hour it is already bright enough. And he extends the midday problem beyond the sixth hour to include the seventh as well. What remains, according to him, is the fourth and fifth hours before midday and the eighth and ninth hours after it.
The Rav rules that the halachah follows Rebbi Yehudah. In summary: Rebbi Meir allows six hours, adding the third and the seventh; Rebbi Yehudah allows only four, the fourth, fifth, eighth and ninth. Both agree that the sixth hour is excluded, that the earliest morning is excluded, and that the hours after the ninth are excluded. In practice, then, it is in the fourth, fifth, eighth and ninth hours of the day, and only then, that a kohen may look at a nega and pronounce upon it.