משנה על נגעים 2:3

If the two hairs were black at the root and white at the tip he is clean. If they were white at the root and black at the tip he is unclean. How much whiteness must there be? Rabbi Meir says: any amount. Rabbi Shimon says: enough to be cut with a pair of scissors. If it was single at the root but split at the tip, and it looks like two hairs, he is clean. If a bright spot had [two] white hairs or black hair he is unclean, for we are not concerned that the place of the black hair lessened the space of the bright spot, since the former is of no consequence.