Midrash su Levitico 14:16

7) "and he shall pour onto the Cohein's left palm": It is a mitzvah (for one Cohein) to pour into another Cohein's palm; but if he pours into his own palm, it is valid." (Vayikra 14:16) ("And the Cohein shall dip his right finger from the oil on his left palm, and he shall sprinkle from the oil with his finger seven times before the L–rd.") "And he shall dip": and not wipe (the palm).

8) (Vayikra 4:6): "And the Cohein shall dip his finger into the blood.": He shall dip and not wipe, (against the wall of the vessel. It must contain enough blood for him to "dip" his finger into it.) "And he shall dip … and he shall sprinkle (seven times"): for every sprinkling, a dipping (and not one dipping for all the sprinklings.) "his finger": "his finger" is written here and elsewhere (Vayikra 14:16, in respect to a leper). Just as "his finger" there is the most dexterous (i.e., the index finger) of the right hand, so, "his finger" here.

11) (Vayikra 16:14) ("And he shall take the blood of the bullock, and he shall sprinkle with his finger on the face of the kaporeth (the ark cover) to the east; and before the kaporeth shall he sprinkle seven times of the blood with his finger.") "And he shall take the blood of the bullock": He takes it from the one who was stirring it. "and he shall sprinkle": and not let it drip (from his finger). "and he shall sprinkle": and not fling. "his finger": "his finger" is written here and elsewhere (Vayikra 14:16) in respect to a leper). Just as "his finger" there is the most dexterous, i.e., the index finger of the right hand, so, "his finger" here.