Bourne's third and last major trip was perhaps his most ambitious; consisting of a six-month journey in the Himalayas with the goal of reaching and photographing the source of the Ganges. He left Simla on 3 July 1866, in the company of Dr. G.R. Playfair (brother of the famous English politician Dr. Lyon Playfair), and travelled with him through Kulu and Lahaul, over the Kunzum Pass into the Spiti valley, where they later parted company. Bourne then continued on alone (except for his forty porters!); over the Manirung Pass, where he took spectacular views of the 18,600 foot high pass; which held the record for the highest altitude photographs that had yet been taken for twenty years. Thence, down to the junction of the Spiti and Sutlej Rivers and on to Sungnam and the Buspa Valley. He then climbed up over the Neela Pass, and down into the Upper Ganges Valley, where he journeyed on up to the Gangotri Glacier. There he went on to photograph one of the prime sources of the Ganges, as it issued from the mouth of the glacial ice cave at Gaumukh. His return journey took in Agra, Mussoorie, Roorkee, Meerut and Naini Tal, and he arrived back in Simla, again in time for Christmas! He wrote extensively about his travels in the Himalayas (one of the very few photographers in India to do so), in a long series of letters, which appeared in The British Journal of Photography, between 1863 and 1870.