See caption
Squint in the wall of the north aisle chapel, St Nicholas’s Church, Walcot, Lincolnshire, with a view of the high altar in the chancel beyond.
Wikimedia Commons

A hagioscope, also known as a squint or loricula, is an obliquely cut opening in the masonry wall of a Christian church to allow a limited view of the high altar from side-chapels or aisles. Its purpose was to allow celebrants in the side chapels to suspend their own services when the parish priest at the high altar reached the point of consecrating the eucharist, known as sacring, so that consecrations did not take place simultaneously.[1]

It has often been suggested that the squint allowed lepers to view the Mass without mixing with the congregation in the naveCentral part of a church, used by the laiety.,[2] and with that purpose in mind it is sometimes called a lychnoscope.[3] But during the medieval period the congregation would have been prevented from seeing the celebration of the Mass by a pulpitum, so the suggestion is almost certainly without merit.[4]