L E C T U R E S A T T H E U N I V E R S I T Y O F M A D R I D 8 7 5 TRANSLATION SPECIAL T HEORY OF RELATIVITY With the hall filled by the many people eager to hear from the lips of its discoverer the theory that had so profoundly revolutionized the bases of physical science, Professor Ein- stein was greeted upon entering by a great salvo of enthusiastic applause. As long as these signs of affection lasted, we noted many personalities entering and taking their seats as rep- resentatives of the governing, aristocratic and scientific classes. Among others, whom we regretfully do not mention, there were Sres Maura, Salvatella, Gimeno, the Count of Grove, Gascón y Marín, the dean of the faculty of sciences Octavio Toledo, and Señores Cabrera, Plans, and P. Enrique de Rafael, well-known for their relativistic publications. At a conve- niently situated desk, we saw Professors Carrasco, Palacios, Lorente de Nó, translator of Einstein’s book, and Dr. T. Rodríguez Bachiller taking notes for the publication of the lec- tures on behalf of the Faculty of Sciences. There fell a solemn silence in which we heard the eager pounding of impatient chests. The professor of mathematical physics, Señor Carrasco, introduces Professor Einstein in a brief but expressive speech, and Einstein begins his lecture. With a relaxed mien, sure into- nation, and slow diction, which eases the comprehension of his French for little-accus- tomed ears, he faces his audience, looking at it with the noble features of a genius, as if he wished to imprint upon them the ideas which inspired him. He says: “In the first place I must excuse myself for two reasons: first, because I cannot address you in your beautiful language, and second because I do not speak well the one in which I must address you.” He outlines the three lectures, proposing to devote the first to special relativity, the sec- ond to general, and the third to problems recently arisen, noting that the first two can be followed with elementary mathematical knowledge, while the third would be harder to un- derstand, inasmuch as he would have to use a more complicated mathematical algorithm, absolute differential calculus. “Now I will say something about the general method of special relativity. The theory of relativity is a deductive theory based on two experimental facts, being in this sense analo- gous to thermodynamics, which is also based on two experimental facts, and its whole ed- ifice is constructed without any additional empirical data.” He pointed out that the theory was derived from research into the optics of moving bod- ies and from the application to such phenomena of the Maxwell-Lorentz equations, equa- tions which he says should be considered the most likely expression of the physical content of our knowledge and which show that light must have a constant velocity in empty space independent of the motion of bodies or of its color. Such a simple and completely general law seems as if it ought not to raise any difficulties, and yet, it did of such a nature that one had to turn to the relativity of motion, the general concept that forms the entire theory of relativity, to resolve them.