THE problem noticed by Mr. Comstock in NATURE of November 19, 1908, is an interesting one, but I do not see how the “laws of electricity and conservation of energy require in themselves the discrete structure of electricity or the association of electricity with matter.” The electromagnetic field produced by a uniform spherical sheet of electricity, unassociated with matter expanding under its own repulsion, is not zero, but indeterminate. The total energy of the system remains finite and constant, while the velocity of expansion is that of light. Thus perfect uniformity of electricity, together with isolation, is not incompatible with the laws of electricity and conservation of energy. The indeterminateness of the electromagnetic field will, of course, surprise no one who is willing to start with a distribution of electricity differing infinitely little from that of perfect uniformity, arranged as a sheet differing infinitely little from spherical, and expanding in surroundings departing infinitely little from the symmetrical.