Henry Knowles, pharmacist and medical practitioner, obtained a certificate from the Pharmaceutical Society of Great Britain and was licensed to practice as a chemist and druggist in the Cape Colony in April 1850. He plied his trade in Grahamstown, where in 1859 he advertised Knowles's liquid to counter infection - a product prepared by him to be applied to the face and hands, or sprinkled on the floor, to prevent the spread of contagious or infectious diseases in the home. During the eighteen-sixties he proceeded to Scotland for further study, qualifying in 1867 as a licentiate of the Royal College of Physicians of Edinburgh (LRCP Edin), a licentiate of the Faculty of Physicians and Surgeons of Glasgow (LFPS Glasgow) and a licentiate of the Society of Apothecaries (LSA). Returning to Grahamstown he was licensed to practice medicine at the Cape in October 1868.
Knowles was elected a member of the Albany Natural History Society in March 1869. In 1887 he patented the "Illuminated hydro-speculum", an instrument of his own invention for viewing objects under water. It consisted basically of two 1,8 m long, tapering pipes, fixed side by side, the wider (bottom) end of each sealed by thick plate glass. One pipe had a reflecting inner surface and a light source at the narrow end, serving to illuminate the underwater scene. The other had a black inner surface and a small field-glass mounted at the narrow upper end. The instrument could be fixed to a boat with the wider ends of the pipes in the water, to inspect harbour works and other submerged objects, both by day and at night.
In 1893 Knowles was still living in Grahamstown, but by 1897 his address was given as Klipdam - presumably the post office by that name north of Kimberley. He may well have died or retired by this time, as he did not confirm his particulars for the medical directory of 1897. Three persons named Henry Knowles died in the Cape Colony around this time, namely in 1888, 1895 and 1903. Dr Henry was probably either the second or third of these.