Even if we suppose it to be sensitive to rays unknown to the ordinary retina, this extraordinary sight could not be the sense that warns the butterfly at a distance and brings it hastening to the bride.  Distance and the objects interposed make the suggestion absurd.

Moreover, apart from illusory refractions, of which there is no question here, the indications of light are precise; one goes straight to the object seen.  But the butterfly was sometimes mistaken:  not in the general direction, but concerning the precise position of the attractive object.  I have mentioned that the nursery on the other side of the house to my study, which was the actual goal of the visitors, was full of butterflies before a light was taken into it.  These were certainly incorrectly informed.  In the kitchen there was the same crowd of seekers gone astray; but there the light of a lamp, an irresistible attraction to nocturnal insects, might have diverted the pilgrims.

Let us consider only such areas as were in darkness.  There the pilgrims were numerous.  I found them almost everywhere in the neighbourhood of their goal.  When the captive was in my study the butterflies did not all enter by the open window, the direct and easy way, the captive being only a few yards from the window.  Several penetrated the house downstairs, wandered through the hall, and reached the staircase, which was barred at the top by a closed door.

These data show us that the visitors to the wedding-feast did not go straight to their goal as they would have done were they attracted by any kind of luminous radiations, whether known or unknown to our physical science.  Something other than radiant energy warned them at a distance, led them to the neighbourhood of the precise spot, and left the final discovery to be made after a vague and hesitating search.  The senses of hearing and smell warn us very much in this way; they are not precise guides when we try to determine exactly the point of origin of a sound or smell.

What sense is it that informs this great butterfly of the whereabouts of his mate, and leads him wandering through the night?  What organ does this sense affect?  One suspects the antennae; in the male butterfly they actually seem to be sounding, interrogating empty space with their long feathery plumes.  Are these splendid plumes merely items of finery, or do they really play a part in the perception of the effluvia which guide the lover?  It seemed easy, on the occasion I spoke of, to devise a conclusive experiment.

On the morrow of the invasion I found in my study eight of my nocturnal visitors.  They were perched, motionless, upon the cross-mouldings of the second window, which had remained closed.  The others, having concluded their ballet by about ten o’clock at night, had left as they had entered, by the other window, which was left open night and day.  These eight persevering lovers were just what I required for my experiment.