YOUR readers are indebted to Mr. Bonacina's letter in NATURE of April 7 for a very clear statement of a fundamental point in the relativity controversy, and it is important that the views held with regard to it should be clearly understood. The issue is stated concisely in the sentence "the relativists seem now to indicate that space, instead of being conditioned by matter, is itself the foundation of matter and physical forces". Now it seems clear that if any relativist expresses himself in terms like these he cannot be regarding space as mere emptiness or as the arbitrary co-ordinate system of the pure mathematician; for him it is the substitution of matter, light, and electric force—that is to say, it is the thing which most of us call aether. Since it is not matter, it has not (and we ought not to expect it to have) the material properties of density, elasticity, or even velocity; but it has other dynamical attributes, measured by tensor-expressions, which stand in much the same relation towards it that mass and strain do towards matter. It is, in short, a physical medium. It is sometimes stated that the relativity theory does away with the aether; the defence of this statement must be left to those who make it; I do not think it is the view of Prof. Einstein. It seems more reasonable to say that relativity has added to the importance of the aether by enlarging its functions.