Experiments have become, if not the sole, yet the most definite, test of intuition. I need only recall the observations of the solar eclipse of 1919, which were of an experimental character inasmuch as they used apparatus to question Nature. To the world generally, they gave the irrefutable confirmation of Einstein's Theory of Gravitation, but not to Einstein himself, whose intuition felt itself so certain that the confirmation was a mere matter of course. But this is not the average case; in many cases the intuition of the discoverer appeals to experiment as a judge of great authority, who is to confirm, reject, or correct. Let us take some examples of cases in which the intensity and the value of intuition were measured by the experimental results. Benjamin Franklin's Kite Experiment may be taken as a classical instance. Here is a man in whose head the idea takes root that lightning and electricity are one and the same thing. Innumerable persons before and after his time might have hit on the same idea, which is now the common knowledge of children. Yet, a single man had to appear who became aware of this pre-formed fact and who simultaneously thought out a method of putting it to proof. In 1752 he constructed a kite, sent it up into the clouds during a storm, and caught up sparks on the ground by a metallic contrivance, and, as d'Alembert so aptly described it to the French Academy: "Eripuit coelo fulmen ..." He wrested the lightning from the heavens. Jupiter Tonans illuminated a great discovery, a mighty intuition which had entered like a lightning stroke into the brain of a discoverer. This case would be classical, were it not that nine-tenths of it is based on legend. Franklin was by no means the first who had this intuition, and his experimental test was so full of faults that it was within an ace of failing. Franklin used a dry thread of hemp, which he thought to be a conductor, but which became a conductor only after it had been made wet by rain. Till that moment the exhibition of sparks on the ground had been poor enough, and little was wanting for Franklin to give up his attempt and confess that he had been inspired, not with an intuition, but with a hallucination.