The right transept arm is lighted by three lancets, which show scenes from the life of our patron saint:
- In the central lancet, Thomas stands above an example of his doubting taken from early Church lore. Supposedly, absent from Mary's deathbed, Thomas remained unconvinced of her assumption until, in mocking reproof, Mary dangled her sash from heaven. The crown, palms, and arrows at the bottom of the lancet are traditional symbols of martyrdom.
- In the left lancet, the former doubter probes Christ's wounds; above, he baptizes three kings (in one tale, the three Wise Men) on his travels in India. At the bottom of the lancet, the five roses stand for Christ's wounds, and the eleven lighted candles symbolize the apostles, sans Judas.
- The right lancet recalls how Thomas was paid to build a palace, but gave the money to the poor; later, he was slain with spears near Madras. The symbols at the bottom of the lancet are of a cross raised on a mountain (evangelism), spears (martyrdom), and a carpenter's square for the construction of the palace.
