I must add that Einstein himself is fond of experimenting, and has had much success in experimental work. The amount of advice and encouragement that he has given, and still gives, to many workers in this field is very considerable. But he does not practise experimental work regularly, and remarked that he is obliged to appeal to outside help for certain practical tests. There are specific experimental geniuses, whose activity assumes the happiest and most fruitful form when it supplements that of the theorist and fertilizes it.
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.
But to whom then is the glory of this discovery due? This is a difficult point to decide. As early as 1746, that is, six years before Franklin's kite made its ascent in Philadelphia, Professor Winkler of Leipzig had asserted in a dissertation that the two phenomena were identical, and had proved this theoretically; and three years earlier still Abbé Nollet had declared the storm clouds to be the conductors of an electrical induction machine. Almost simultaneously with Franklin, Dalibard, Delor, Buffon, Le Monnier, Canton, Bevis, and Wilson made experiments on an elaborate scale, which far exceeded that of Franklin in their results. To this must be added that the experiment was conducted with evident success only in 1753, when de Romas of Nerac in South France wove a real conductor of thin annealed wire into the kite-string, and succeeded in bringing down a regular thunderstorm with flashes of lightning ten feet long, accompanied by a deafening uproar. It was only then that the track of the inspiration was traced back through time to the Roman Kings, Numa Pompilius and Tullus Hostilius, as the first experimenters with lightning. And then the physicist Lichtenberg sought to furnish a proof that the old Hebrew ark of the Covenant, together with the tabernacle, were nothing other than great pieces of electrical apparatus highly charged with electricity derived from the air; thus the first intuition, and the priority of discovery, would have to be ascribed to Moses or Aaron! And connected with this was the fact, supported by substantial proof, that the Temple of Solomon was protected by lightning-conductors.
I must not omit to mention that Einstein regards this whole chain of proofs stretching back to early times as by no means established, although besides Lichtenberg, other important scholars, such as Bendavid in Berlin and Michaelis in Göttingen, have vouched for their truth. And as it is a matter of electrical relationships, Einstein's doubts cannot be passed over. As far as I recollect, they were not directed against the rough facts in themselves, but against the sense that is construed into them—that is to say, in the case of both the ancient Roman and the Biblical data, the conception of discovery must be excluded, and must be awarded rather to those intellectual efforts which have led to the creation of a method of thought. None the less, we may uphold our statement that in this case, presumed to be classical, neither Franklin nor anyone else is to be claimed as the discoverer or as the central figure in a creative act.
The experimental case of spectral analysis is incomparably simpler and less open to dispute. It is without doubt a discovery of fundamental importance bearing all the characteristics of originality, for no predecessors are discernible. I have always felt a little dissatisfied with the fact that it required two men to think it out, that a duo of minds was necessary for one act of thought which appears quite uniform, elementary and inseparable from the intuition of a single mind. But it seems possible that tradition has not handed the facts down to us faithfully, and that the two men, with a unanimity arising from their partnership in work, combined their results, which were not, at the beginning, of a dual character. This possibility became clear to me from a remark of Einstein which made it plain to me that the conjunction Kirchhoff and Bunsen is to be taken as denoting Kirchhoff and then, after a pause, Bunsen in the next breath! But if we discard this question of unity or duality, we are left with the fact that the idea of a spectral analysis occurred to some one (as a result of preceding optical experiments with Fraunhofer lines), and was fully confirmed by later experiments. Only fully confirmed? No, the classic rank of this case manifested itself in a much more triumphant manner, for it is impossible that the intuition of Kirchhoff and Bunsen could have grasped the whole significance and range of their discovery even after they had made it their own.