23 February 1863
John Speke and James Grant, on their return from an expedition in Africa announced that Lake Victoria is the source of the Nile River. The expedition was Speke’s second attempt and was meant to put to rest all doubt on his earlier findings. Speke’s first expedition to Africa was in 1856 when he accepted an invitation by Richard Burton to search for the great lakes of Africa. Their expedition would be a difficult one. They were plagued by disease, and on several occasions had to take alternative routes to avoid tribal wars. During the expedition they were once captured, attacked and severely injured. At one point Speke had to leave Burton behind and go in search of the great lake, which he found and named Lake Victoria. After studying the lake, Speke concluded that it was the source of the Nile River. However, when he returned to England and reported his findings, many, including Burton, were not convinced.
References

BBC History," John Hanning Speke (1827 - 1864)", from:BBC History, [online] Available at: www.bbc.co.uk [Accessed on 14 February 2013]