That I did not do so was due to a fortuitous observation.

Chance often has a surprise in store which sets us on the right road when we have been seeking it in vain.
One afternoon, while trying to determine whether sight plays any part in the search for the female once the males had entered the room, I placed the female in a bell-glass and gave her a slender twig of oak with withered leaves as a support.

The glass was set upon a table facing the open window.

Upon entering the room the moths could not fail to see the prisoner, as she stood directly in the way.

The tray, containing a layer of sand, on which the female had passed the preceding day and night, covered with a wire-gauze dish-cover, was in my way.