Searching here is done using the surname and up to two first names. If you enter two first names, each one is used, together with the surname, to find matches in the records. This means, for example, that a search for "James Thomas Lockyer" may result in the selection of "James Lockyer", "Thomas Lockyer", "James Thomas Lockyer" or even "Thomas James Lockyer". In practice, any one of these might turn out to be the person you are seeking.
The search algorithm is not able automatically to substitute variations of a first name. For example, if you enter the name "Tom", it will not find "Thomas". You are advised, then, to try such known variations if your search does not immediately yield the person sought.