Alfred Russell Wallace accepted the challenge. Wallace was a prominent scientist who contributed to many fields, including the theory of natural selection, but his career began as a surveyor's apprentice. His terms for replicating the experiment involved two additions. First, he placed a marked pole midway between the surveyor and the boat at its farthest point. With the telescope, the mark, and the mast all at the same height, he predicted that if the Earth was flat, all three objects would fall in a straight line; whereas, if it was round, the distant flag would be shown to have dropped below the midpoint marker. Second, he said the estimate of the drop should account for the atmospheric refraction of light. This is the principle that light curves as it travels through different densities of the atmosphere, which creates an effect where distant objects like the setting sun appear to be visible even when they have dropped below the horizon.