Numerous optical centres are situated throughout the country according to the distribution of troops. There are also sub-centres where their formation is justified by the number of troops in the area. These are visited by ophthalmic specialists from the main centres. A similar service exists in the Royal Air Force, and the facilities provided by the two Services are available for use by either where convenience is served. In this way a more efficient service is provided than would be possible if travelling opticians took the place of the static centres, and, in general, less transport is used.