Tractate Bechoros, chapter 7, mishnah 3. The mishnah continues to list aesthetic flaws that cause a person to look unlike a typical descendant of Aharon, and therefore he is unfit for the service. The main focus of our mishnah is on blemishes around the eyes.
"Eizehu charum":
The mishnah begins by stating that a charum is disqualified. The word "charum" means something destroyed, and we need to understand what is destroyed in this person. The mishnah explains: "Eizehu charum? Shekochel shetei einav ke'achas" - What is a charum? One who can paint both his eyes at once. He has no bridge on his nose, and therefore he can pass a makeup brush over both eyelids simultaneously without bumping into the bridge of the nose. The bridge of the nose is completely missing, and this person's appearance is so abnormal that he cannot serve.
This is the opinion of Rabbi Yosi in the Gemara. The Sages maintain that a complete absence of the bridge of the nose is not necessary: even if the bridge is merely sunken inward, to the extent that an uninhibited child would say to his mother "Look at this man, his nose is so sunken" - that is already a problem, and this is the halachah.
Blemishes in the position and appearance of the eyes:
"Shetei einav lemaalah" - Both his eyes are too high on his head, meaning the eye sockets are located approximately where his forehead should be.
"Shetei einav lematah" - His eyes are very low, to the point that the eye sockets are located where his cheeks should be. His appearance is strange, and he cannot serve.
One eye above and one eye below - One eye is normal and the other is too high, or one eye is normal and the other is too low, so the eye sockets are asymmetrical and his appearance is strange enough to disqualify him from the service.
"Haro'eh es hacheder ve'es ha'aliyah ke'achas" - One whose eyes are cross-eyed. Medically, this is hypotropia: the eye muscles are not strong, and therefore one eye appears to look up and the other straight ahead. It turns out the exact definition is the opposite, where the eye looks down or to the side, but in any case, such an appearance, if it is very strange, presents the same problem. The mishnah takes the case where one eye looks up and the other straight or down - this is a problem.
"Sachei shemesh" - The word is related to "sukkah" and "sechach," meaning to cover and shade. This person shades himself from the sun because his eyes are sensitive and he cannot be exposed to direct sunlight; he is always blinking or shutting his eyes tightly in the light. His appearance is strange, and he cannot serve.
"Hazugdos" - The word is composed of two words meaning pair, double, or twin: "zug" in Hebrew, and "dos," which means two in Latin (like counting uno, dos, or like the word deuce in a card game, meaning the number two). This means that his limbs do not match properly as a pair. The main example is heterochromia - two eyes of different colors: a blue eye and a green eye, or a blue eye and a black eye, and this appearance is very strange. Fundamentally, however, all humans are created with bilateral symmetry, and everything comes in pairs on the face and body: two ears, two eyes, two nostrils, two hands, and two legs. Therefore, any prominent asymmetry - one eyebrow without the other, one ear without the other, or one very long hand and the other very short - all those organs that were supposed to be identical and parallel but are not, are included in the general category of zugdos.
"Vehatziran" - A person whose eyes are particularly watery, a condition called epiphora. His eyes do not drain properly, and the problem is in the tear ducts, so he is constantly tearing excessively. He looks as if he is always crying, and his face is wet even though he is not crying. This is also a problem.
"Veshenashru risei einav pasul mipnei maris ha'ayin":
Here we are dealing with a different matter. One whose eyelashes have fallen out, leaving only small stumps where the lashes used to be, his appearance is not at all pleasant, and therefore he is disqualified because of maris ha'ayin. However, the term "maris ha'ayin" does not have its usual meaning here: in other places it means what people will say, lest they get the wrong impression when seeing him. Here it means quite literally - the way he looks to the eyes.
We are therefore dealing with an assessment that is solely miderabanan: what is an appearance that is so unaesthetic that it is unfitting for him to serve. This is the condition of this person, whose eyelashes fell out leaving small stumps and a strange appearance, and according to the Sages, he should not serve. This is a rabbinic law, and therefore if he did serve, he does not violate a positive or negative commandment, and his service is valid bedieved. But lechatchilah, since his appearance is strange, unusual, and unpleasant - this is maris ha'ayin, and he should not serve.