He was considered the foremost optician in Britain, and was noted for his skill in mirror making and other optics for astronomical telescopes.
He partnered to create the firm Cox Hargreaves and Thompson, with first premises under the embankment of the mainline station near Preston Park, Brighton, Sussex.
Later, moving to the much better environment for really accurate grinding and surface continuity at the large tunnel layout (Deep Level Air Raid Shelter No.
Cut deep into a chalk hill, these brick-lined tunnels had an all-year round constant temperature and absolutely no vibration.
Hargreaves trained telescope-maker Norman Fisher (1936-1994) in mirror making at the Cane Hill tunnels.