he is asking for an experiment that anyone can do to prove to themselves that earth is not flat

As one focuses further out, one focuses on more light; hence, more “objects” occupy its “field of vision” and, therefor, increase the “object”-density thereof. However, “the disappearance of” (notbatman) occurs when light from a body that was, previously, in view reaches neither the cones nor the rods of the retina (e.g., for the obstruction thereof).