I conceive of it as an exhalation which is given off during courtship and gradually saturates whatever is in contact with the motionless body of the female.
If the bell-glass was placed directly on the table, or, still better, on a square of glass, the communication between the inside and the outside was insufficient, and the males, perceiving no odour, did not arrive so long as that condition of things obtained.

It was plain that this failure of transmission was not due to the action of the glass as a screen simply, for if I established a free communication between the interior of the bell-glass and the open air by supporting it on three small blocks, the moths did not collect round it at once, although there were plenty in the room; but in the course of half an hour or so the feminine alembic began to operate, and the visitors crowded round the bell-glass as usual.
In possession of these data and this unexpected enlightenment I varied the experiments, but all pointed to the same conclusion.

In the morning I established the female under the usual wire-gauze cover.

For support I gave her a little twig of oak as before.

There, motionless as if dead, she crouched for hours, half buried in the dry leaves, which would thus become impregnated with her emanations.
When the hour of the daily visits drew near I removed the twig, which was by then thoroughly saturated with the emanations, and laid it on a chair not far from the open window.