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Johnston first arrived in Hong Kong in 1898 at the age of 24. In 1919 he was recommended as tutor to the teenage Emperor Puyi, a post he held for six years, becoming both friend and adviser.

He always kept his Box Brownie camera handy, describing himself in one letter as a "keen snapshotter", as he moved in the highest circles in imperial China, The Scotsman reported.

Many of Johnston's photographs of a lost era were burned when his mistress destroyed his papers after his death in 1938. But he sent others to his friend, James Stewart Lockhart. For decades, some 4,000 photographs have been kept in boxes in an attic, carefully stored and cataloged. They are now on permanent loan to the Scottish National Photography Collection.

Photographs from the archive will go on show at the Scottish National Portrait Gallery May 20.