“Ah!” he replied, with a smile, “I was not aware you had moved.  Experience has taught me about where to look for the large city you call Paris, on the side of Earth that is now exposed to us, and then by systematic search I soon located the response of your instrument.

“As our observations of Earth with projected light-rays have been carried on for seven hundred years, it will be necessary to give you an outline of our history and the progress of science covering that time.  This will not only be of interest as a forecast of your own world’s future, but will also prove of the greatest value to you, if you decide to visit this planet, an undertaking which I am convinced lies within your power.”

His words wrung an exclamation of astonishment from my lips, but, as though not wishing to be interrupted, he went on: 

“Seven hundred years ago, a power derived from that substance known on Earth as radium, was discovered on Mars.  This power was found to be capable of projecting light rays almost instantaneously through space for inconceivable distances, at the same time preserving their integrity to such a remarkable degree that they would reach the farthest planets without diffusion or diminution.  Thus my image, thrown upon the instrument before me, is conveyed to Earth in light-waves by this flow of super-radium with such tremendous speed as to be practically instantaneous; these are received in your instrument, which is responsive to the flow of super-radium, in the same condition as when they left Mars, consequently depicting the image life-size.

“Having come in contact with another body in the heavens, this outward-flowing current of super-radium is changed to an inward-flowing current.  In making this change it frees the light-waves it conveyed from Mars, and retains the light-waves of the objects about it, which is merely repeating its performance upon leaving Mars.  These light-waves of objects on another globe it now conveys on its return journey to Mars, entering a receiving instrument and depicting the objects therein life-size.

“Possessing rays invisible to the human eye, except when agitated by a substance of its own nature, daylight on a planet becomes an entirely unnecessary adjunct to observations made with super-radium, and we are able to explore the dark side of planets and other heavenly bodies, just as effectually as those illuminated by the sun.

“Thus have we, for seven hundred years, been able to study the country, cities, streets, and people of Earth.  And not only did we note a remarkable similarity in the people, buildings, and scientific progress to early Martian ages, but, by the advertisements, placards, and other street signs we were able to learn the principal languages spoken on your planet, and these were found to correspond in a remarkable degree to those in use on Mars, before conditions on our planet made the adoption of a composite language an absolute necessity.  And undoubtedly these same conditions in due time will face the people of Earth.”