TT #88 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for the requirement to see the full appearance at one time.
Sifra — Vayikra 13, Midrash Sefer
English: “And he shall examine it” — he must see it all at once. If it is at the tip of the nose or finger and slopes in both directions, it is not impure.
Textual limitation
The priest must be able to see the entire relevant appearance at one time; a mark spanning a rounded edge in both directions does not meet that requirement.