Dr. Balthazar is a lawyer, but why does Shylock call him noble judge?
Dr. Balthasar, also known as Portia in disguise, is a lawyer who has a doctorate degree in law. He is well-versed in the law because the letter from Bellario which is sent to validate him before the Duke says, "We turned o'er many books together" (IV.i.154). The Duke had requested Bellario to come to the hearing as a consultant and mediator between the two parties, to clarify the law as written in the books, and to help the Duke pass judgment.
Once Portia is accepted by the Duke, she starts analyzing the situation and speaks publicly with Shylock. She soon finds out that no negotiations will change his mind from claiming a pound of Antonio's flesh as payment for his bond. They verbally wrestle back and forth for a long time about the legality of the contract and whether or not Shylock will give mercy. As Portia realizes that Shylock won't budge, she says that the law is on his side and must be fulfilled if he won't negotiate. When Portia tells Antonio, "You must prepare your bosom for his knife," Shylock exclaims, "O noble judge, O Excellent young man!" (IV.i.240-241).
Shylock makes this exclamation the moment he feels Balthasar sides with him and that he will finally be able to follow through with taking a pound of Antonio's flesh. He is also claiming vocally that Balthasar has ruled correctly, which would make him a "noble judge" in Shylock's eyes. It's almost as if he's saying that this young lawyer can see things his way, so why can't anyone else? Up until this point, Shylock has struggled against Christians, and he finally thinks that he will be able to enact revenge. The Duke still has the final say on the matter, but since he has not interrupted Balthasar during this discussion, it seems that he supports it—or at least he sees that the law must be fulfilled. Thus, Shylock is declaring that Balthasar understands the law correctly and is a worthy judge to pass judgment on this case.