Another rather common objection is presented by certain persons apparently scientific. Confounding experience with observation, they imagine that a natural phenomenon, in order to be real, ought to be able to be produced at will, as in a laboratory. After this manner of looking at things, an eclipse of the sun would not be a real thing, nor a stroke of lightning which sets fire to a house, nor an aerolite that falls from the sky. An earthquake, a volcanic eruption, are phenomena of observation, not of experiment. But they none the less exist, often to the great damage of the human race. Now, in the order of facts that we are studying here, we can almost never experiment, but only observe, and this reduces considerably the range of the field of study. And, even when we do experiment, the phenomena are not produced at will: certain elements, several of which we have not yet been able to get hold of, intervene to cross, modify, and thwart them, so that for the most part we can only play the role of observers. The difference is analogous to that which separates chemistry from astronomy. In chemistry we experiment: in astronomy we observe. But this does not hinder astronomy from being the most exact of the sciences. Mediumistic phenomena that come directly under the observation, notably those I have described some pages back, have for me the stamp of absolute certainty and incontestability, and amply suffice to prove that unknown physical forces exist outside of the ordinary and established domain of natural philosophy. As a principle, moreover, this is an unimpeachable tenet.[5] I could adduce still other instances, for example the following: 7. During seance experiments, phantoms often appear,--hands, arms, a head, a bust, an entire human figure. I was a witness of this thing, especially on July 27, 1897, at Montfort-l'Amaury (see Chapter III). M. de Fontenay having declared that he perceived an image or spirit over the table, between himself and me (we were sitting face to face, keeping watch over Eusapia, he holding one of her hands, and I the other), and I seeing nothing at all, I asked him to change places with me. And then I, too, perceived this spirit-shadow, the head of a bearded man, rather vaguely outlined, which was moving like a silhouette, advancing and retiring in front of a red lantern placed on a piece of furniture. I had not been able to see at first from where I sat, because the lantern was then behind me, and the spectral appearance was formed between M. de Fontenay and me. As this dark silhouette remained rather vague, I asked if I could not touch its beard. The medium replied, "Stretch out your hand." I then felt upon the back of my hand the brushing of a very soft beard.