The correct law of refraction, sin i = μ. sin r, is usually assumed to have been first discovered by Snell in 1621, although he did not express it in this form. The astronomer Kepler laboured hard to discover it, but in vain, and Whewell in the History of the Inductive, Sciences says that it is strange that he should have failed, when the law is so simple. There is, however, a good reason for his want of success.