Hall therefore decided to make a convex lens out of crown glass and a concave lens out of flint glass, designed in such a way that the two would fit together to form a single
biconvex lens. The crown glass would spread out the colors, while the flint glass would bring them together again without neutralizing all the magnifying value of the crown glass.
In 1949 three experienced observers in the United States independently reported that only one-third of "the dark face glowed over an area in the form of a
biconvex lens, with one surface along the terminator and the other some way within the dark limb." Several observers at widely separated locations in Britain independently reported that the ashen light was unusually conspicuous on the evening of March 17, 1953.