Megillah, Chapter 4, Mishnah 6. This mishnah deals with the question of who may serve the congregation in the various roles in the synagogue, and which roles are closed to someone who has a certain deficiency - whether a deficiency of age, a deficiency in outward appearance, or a deficiency in one of the senses.
A Minor:
A minor is a child who has not yet reached the age of bar mitzvah. The mishnah states: "Katan korei baTorah u'metargem" - a minor reads from the Torah and translates:
"Korei baTorah" - he is permitted to be called up and read from the Torah. The Rambam writes that according to one of the Geonim this is permitted only after the third aliyah, since the first three aliyos are the minimum required number, while the additional aliyos are added on account of other circumstances.
"U'metargem" - he may translate the Torah reading into Aramaic.
What these two have in common: in neither of them does the minor fulfill anyone else's obligation, nor does he intend to do so.
By contrast, the mishnah states: "Aval eino poreis al Shema, v'eino oveir lifnei ha'teivah, v'eino nosei es kapav" - but he does not lead the congregation in the berachos of Shema, he does not pass before the ark, and he does not raise his hands:
"Eino poreis al Shema" - he cannot fulfill the congregation's obligation in the berachos of Shema, since he does not have the ability to discharge others' obligations.
"V'eino oveir lifnei ha'teivah" - he cannot serve as the chazzan for Shemoneh Esrei, that is, for the repetition, which includes Kedushah, for the same reason.
"V'eino nosei es kapav" - he does not go up to the duchan. This means that he does not go up alone when he is the only kohen present; but he may join together with other kohanim.
A Poteiach (One in Torn Garments):
From here the mishnah moves on to a person with a different kind of deficiency: the poteiach, one whose clothes are torn and whose appearance is not dignified.
"Poreis al Shema u'metargem" - he is permitted to fulfill the congregation's obligation in the berachos of Shema and also to translate, since in these two roles he does not stand in a prominent place in the synagogue.
"Aval eino korei baTorah, v'eino oveir lifnei ha'teivah, v'eino nosei es kapav" - in all of these he may not serve, because when reading from the Torah, when standing as chazzan for Shemoneh Esrei, and when raising his hands for the priestly blessing, he stands in a prominent place, visible to all.
A Blind Person:
"Suma poreis es Shema u'metargem" - a blind person may fulfill the congregation's obligation in the berachos of Shema, and may also translate.
As for reading from the Torah, he certainly cannot serve as the reader, since he would be reading by heart, whereas one is obligated to read from the written text; and there is a discussion whether he may recite the berachah and have someone else read for him. Regarding passing before the ark as well, to recite Shemoneh Esrei and the repetition, opinions are divided. Some understand that he is certainly permitted to do so, and that the wording of the mishnah is not precise and includes this case as well.
The Opinion of Rabbi Yehudah:
"Rabbi Yehudah omer: kol shelo ra'ah me'oros mi'yamav eino poreis al Shema" - Rabbi Yehudah says: anyone who has never seen the heavenly lights in his life does not lead the congregation in the berachos of Shema. One who was born blind and has never seen light cannot fulfill the congregation's obligation in the berachos of Shema, since he has never benefited from light and therefore cannot recite a berachah over it - and the berachah over light is one of the berachos we recite among the berachos of Shema. The Tanna Kamma, by contrast, holds that even he benefits from light.
The Gemara brings a beraisa on this subject: Rabbi Yosei said that he never understood the verse in the rebuke in Sefer Devarim, which compares the situation to a blind man groping in the dark. What difference does it make to a blind man whether it is dark or not? Until he met a blind man holding a torch in his hand. He asked him: "Why are you holding a torch?" The man answered: "When the torch is in my hand, people see me and can warn me about obstacles; but without it, those who can see are unable to tell me anything." From this he understood the function of light: even though the blind man does not benefit from the light himself, he benefits from it through others.
In summary: In this mishnah we learned the principle that distinguishes between a role in which the one serving does not discharge others' obligations and a role in which he fulfills the congregation's obligation, and between a role performed in a prominent place and one that is not prominent. A minor reads from the Torah and translates, but does not lead the berachos of Shema, does not pass before the ark, and does not raise his hands alone; the poteiach leads the berachos of Shema and translates, but does not serve in the prominent roles; and the blind person leads the berachos of Shema and translates, while according to Rabbi Yehudah one who has never seen the heavenly lights is excluded - a dispute rooted in the question of whether he benefits from light, as emerges from the beraisa about Rabbi Yosei and the torch.