Some very complex geometrical difficulties are involved in these problems. This is shown by the fact that learned mathematicians have written extensive disquisitions on these curious constructions (for example, Dr. Dingeldey's book, published by Teubner, Leipzig). Einstein had never taken notice of these wonders of the scissors, but when I began to form these strips, to paste them, and to cut them, he immediately became interested in the underlying problem, and predicted in a flash what puzzling chain constructions would result in each case, with a certainty that would lead one to imagine that he had spent days at it. On another occasion a space-problem dealing with dress came up for discussion: Can a properly dressed man divest himself of his waistcoat without first taking off his coat? One would not have dared to confront Copernicus or Laplace with such a problem. Einstein at once attacked it with enthusiasm, as if it were an exercise in mechanics, the body being the object; he solved it in a trice, practically, with a little energetic manipulation, much to the amazement and joy of the beholder, who asked himself: Is this the same Einstein who developed the work of Copernicus and Newton? A little later, perhaps, the conversation centres around some serious point drawn from politics, political economy, sociology, or jurisprudence. Whatever it may be, he knows how to spin out the suggested thread, to establish contact with his partner in conversation, to open up his own perspectives without ever insisting on his point of view, always stimulating and showing a ready sympathy for the subject of discussion and for all the ideas which it crystallizes, the prototype of the scientist, in the mouth of whom Terence put the words: "I am a human being; nothing that is human is alien to me!" CHAPTER IX AN EXPERIMENTAL ANALOGY Forms of Physical Laws.--Aids to Understanding.--Popular Descriptions.--Optical Signals.--Simultaneity.--Experiments in Similes.