Another great influence was Willis Lamb, who had done experiments using the newly developed microwave technique, he had done experiments on excited states of hydrogen, in particular, the first excited state of principle quantum number two. According to Dirac Theory, it should have a... two... fine structure consisting of two levels. One involving an S-electron... no, one having a total spin of one half and the other having a spin of three halves. And the Dirac's theory gave us the exact formula for this... the separation of these two states. So Lamb was able to verify that by microwaves. But he found in addition that there really three states, not two, that there were two states of spin one half, one having orbital momentum zero and the other, orbital momentum one, called, in spectroscopy, an S-state and a P-state. According to Dirac theory, they should coincide. But they didn't. And Lamb not only showed us that they didn't, but they... he also measured how much they differed in energy. They differed by about an energy corresponding to a frequency of 1000MHz, that is, a 1000 million cycles per second. And that is known as the Lamb Shift. Now, there had been vague inklings of that in spectroscopy before, but the resolution of optical spectroscopy was not good enough to make sure. And Lamb's experiment made it perfectly clear that these were two completely separate states, that one was an S-state and one a P-state.