Subcommittee X was formed in 1944 as a result of a decision to divide Subcommittee XVIII into two groups—one on physical properties (now Subcommittee XVIII) and one on optical properties (Subcommittee X). Thus, it is believed appropriate to include in this report of the activities of Subcommittee X a brief reference to the work on optical properties of Subcommittee XVIII prior to 1944.