THE difficulty referred to by Prof. Michelson in NATURE of October 6, 1898, and in subsequent letters, to that in your issue of January 19, involves a disregard of the distinction which it is necessary to make between a quantity which, however small it may at first be taken, is thereafter to be kept fixed, and a quantity which can be or is absolutely zero. To this distinction there is the analogous one between a quantity which is arbitrarily large but still considered as limited, and a quantity which is entirely unbounded.