IN the last century, journalism was a perilous profession. The editors of The Hong Kong Daily Press, The Friend of China and the Hong Kong Telegraph, all spent some time in jail for libel. As what they wrote was sometimes true, their fines were often paid by public subscription. However, aggrieved readers occasionally took the law into their own hands. One of these was Lieutenant John Neptune Sargent of the 95th Regiment. Before there was public street lighting in Hong Kong, every householder was required by law to have a lamp outside his door after nightfall. While on patrol, Lieutenant Sargent noticed that a lantern hanging outside a shop was not burning. The shopkeeper was taken to court. Unfortunately when reporting the case the editor of The Hong Kong Register, Mr Cairns, referred to Sargent as an ''informer'' - not once - but twice. It is unwise to call an Irishman an informer. An infuriated Sargent descended like an avenging angel on poor Mr Cairns. He dragged the editor out of bed and ''struck him several blows on the head with his fist and umbrella.'' ''You shall pay for this!'' screamed Cairns, whereupon ''Sargent returned and gave Cairns a few more whacks with his umbrella.'' On December 6, 1847, Sargent was fined $1,000 for assault. Thirty-five years later, Sargent, now a Lieutenant-General, returned to Hong Kong as the Commander of British Forces. Sargent wanted modern guns. The Colonial Secretary, Lord Derby, instructed the Governor, Sir George Bowen, to advance funds to pay for some howitzers. Bowen foolishly questioned the instruction. Sargent was furious. Perhaps with Mr Cairns in mind, Bowen fled to Japan, where he sought a cure for a suspiciously convenient attack of rheumatism.