TY  - RPRT
TI  - Letter Circular 571: optical instruments, refractometry, and optical glass (superseding Letter Circular 485)
PY  - 1939
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 571
DO  - 10.6028/NBS.LCIRC.571
ER  -