Tiles in 3 grades, best to worse: Cut (tiles tested with
polariscope; sized to 1/100th inch; polished and ground edges; prepared
Commercial (selected prisms; not tested, polished, or ornamented), and
Factory (seconds, minor flaws).
Refraction angle noted as A for canopies, J, K, L, M, N, O,
S and P for normal sash and forilux use.