The first, and not the least difficulty, is to clearly define the subject. The words "wireless telegraphy," which at first seem to correspond to a simple and perfectly clear idea, may in reality apply to two series of questions, very different in the mind of a physicist, between which it is important to distinguish. The transmission of signals demands three organs which all appear indispensable: the transmitter, the receiver, and, between the two, an intermediary establishing the communication. This intermediary is generally the most costly part of the installation and the most difficult to set up, while it is here that the sensible losses of energy at the expense of good output occur. And yet our present ideas cause us to consider this intermediary as more than ever impossible to suppress; since, if we are definitely quit of the conception of action at a distance, it becomes inconceivable to us that energy can be communicated from one point to another without being carried by some intervening medium. But, practically, the line will be suppressed if, instead of constructing it artificially, we use to replace it one of the natural media which separate two points on the earth. These natural media are divided into two very distinct categories, and from this classification arise two series of questions to be examined. Between the two points in question there are, first, the material media such as the air, the earth, and the water. For a long time we have used for transmissions to a distance the elastic properties of the air, and more recently the electric conductivity of the soil and of water, particularly that of the sea. Modern physics leads us on the other hand, as we have seen, to consider that there exists throughout the whole of the universe another and more subtle medium which penetrates everywhere, is endowed with elasticity _in vacuo_, and retains its elasticity when it penetrates into a great number of bodies, such as the air. This medium is the luminous ether which possesses, as we cannot doubt, the property of being able to transmit energy, since it itself brings to us by far the larger part of the energy which we possess on earth and which we find in the movements of the atmosphere, or of waterfalls, and in the coal mines proceeding from the decomposition of carbon compounds under the influence of the solar energy. For a long time also before the existence of the ether was known, the duty of transmitting signals was entrusted to it. Thus through the ages a double evolution is unfolded which has to be followed by the historian who is ambitious of completeness. § 3