Reference for Shevuot 11:9

אימור דשמעת ליה לרבי עקיבא בפתוך בחלוק מי שמעת ליה

Well, then from R'Hanina's statement [we may deduce that R'Akiba enumerates the shades one above the other], for R'Hanina said: To what may R'Akiba's statement be compared? - To four tumblers of milk; into one there fell two drops of blood; into the second, four drops; into the third, eight drops; and into the fourth twelve drops - some say, sixteen drops. They are all shades of white, but one above the other.

Jastrow