Bekhorot 38B

גמ׳ מאי הריס? אמר רב פפא: תורא ברא דעינא. דק תבלול, תנו רבנן: דק משוקע ⁠— הרי זה מום, צף ⁠— אינו מום.

Talmud: The Mishnah’s list of blemishes of the eye included an eyelid [ris] that was pierced, damaged, or split. The Talmud asks: What is the meaning of: The ris? Rav Pappa says: It means the outer line, or barrier, of the eye, i.e., the eyelid. The Mishnah further stated: A cataract, a tevallul. With regard to a cataract, A baraita states: A sunken cataract, i.e., one that is in the eye itself, is a blemish, but a floating cataract, i.e., one that is floating above the eye, is not a blemish.

והתניא איפכא! לא קשיא, הא בשחור הא בלבן. והא: אין מומין בלבן! אלא, הא בדוק לבן, הא בדוק שחור.

The Talmud asks: But isn’t the opposite taught in a baraita, that a sunken cataract is not a blemish whereas a floating one is a blemish? The Talmud answers: That is not difficult, as this first baraita is referring to a cataract in the black portion of the eye, whereas that second baraita is referring to one in the white portion of the eye. The Talmud challenges: But it was taught that there are no blemishes in the white portion of the eye. Rather, both baraitot are referring to the black portion of the eye, and this second baraita is referring to a white cataract, whereas that first baraita is referring to a black cataract.

דאמר רבה בר בר חנה: סח לי יאשיה דמן אושא: דק שחור משוקע ⁠— הרי זה מום, צף ⁠— אינו מום. דק לבן משוקע ⁠— אינו מום, צף ⁠— הרי זה מום, וסימניך: ברקא.

This is as Rabba bar bar Ḥana says: Yoshiya from Usha told me that with regard to a black cataract, if it is sunken it is a blemish, but if it is floating it is not a blemish. Conversely, with regard to a white cataract, if it is sunken it is not a blemish, whereas if it is floating it is a blemish. And your mnemonic to remember this is barka, which is a disease in which white patches float above the eye, and it is considered a blemish.