"But science does not prevent us from asking ourselves whether, after the failure of these attempts, the inhabitants of Venus did not resort to another method of correspondence. How can we avoid thinking, for instance, of the X-rays, whose strictly rectilinear path would allow of the formation of pictures so clear that one could wish for nothing better? In fact it is not impossible that these rays are employed for the emission received on the Meudon screen, though the quality of the light when analyzed in the spectroscope makes the supposition highly improbable. But how are we to explain by means of X-rays _the taking of the terrestrial views_ of which we saw the moving outline on the screen? We know, of course, if we go back to the concrete example to which I referred just now, we know that neither the brothers Montgolfier nor the surrounding landscape emitted X-rays. It is not therefore through the medium of these rays that the Venusians can have secured the instantaneous photographs which they afterwards transmitted to us. "Well, this exhausts all the possibilities of an explanation which can be referred to the present data of science. I declare positively that to-day, in this essay, I should not have dared to venture into the domain of theory and to suggest a solution in which my own labours are involved, if Noël Dorgeroux had not in a manner authorized me to do so. The fact is that, twelve months ago, I issued a pamphlet, entitled _An Essay on Universal Gravitation_, which fell flat on publication, but which must have attracted Noël Dorgeroux's particular attention, because his nephew, Victorien Beaugrand, found my name written among his papers and because Noël Dorgeroux cannot have known my name except through this pamphlet. Nor would he have taken the trouble to write it down, if the theory of the rays of gravitation which I developed in my pamphlet had not appeared to him to be exactly adapted to the problem raised by his discovery?