The records of service of officers commissioned prior to 1901 are arranged either by date of commission or by unit. Not all of the records of service for the period prior to 1901 have survived. All officers are listed in the Army List in our microfilm reading room, so it is possible to get an idea of an officers' career from this.
The Catalogue
For officers' records of service the two key records series are WO 25 and WO 76, and a name index for these two series can be found in the microfilm reading room at The National Archives.