Are there effluvia analogous to what we call odour: effluvia of extreme subtlety, absolutely imperceptible to us, yet capable of stimulating a sense-organ far more sensitive than our own?
A simple experiment suggested itself.

I would mask these effluvia, stifle them under a powerful, tenacious odour, which would take complete possession of the sense-organ and neutralise the less powerful impression.
I began by sprinkling naphthaline in the room intended for the reception of the males that evening.

Beside the female, inside the wire-gauze cover, I placed a large capsule full of the same substance.

When the hour of the nocturnal visit arrived I had only to stand at the door of the room to smell a smell as of a gas-works.

Well, my artifice failed.
The butterflies arrived as usual, entered the room, traversed its gas-laden atmosphere, and made for the wire-gauze cover with the same certainty as in a room full of fresh air.
My confidence in the olfactory theory was shaken.