Arthur Conan Doyle's The History of Spiritualism contains the passage quoted below.
In his presidential address before the British Association at Bristol in 1898, Sir William briefly referred to his earlier researches. He said: Upon one other interest I have not yet touched—to me the weightiest and farthest—reaching of all. No incident in my scientific career is more widely known than the part I took many years ago in certain psychic researches. Thirty years have passed since I published an account of experiments tending to show that outside our scientific knowledge there exists a Force exercised by intelligence differing from the ordinary intelligence common to mortals . I have nothing to retract. I adhere to my already published statements. Indeed, I might add much thereto.
What does "one other interest I have not yet touched—to me the weightiest and farthest—reaching of all" mean? does it refer to a particular subject of presidential address?