The Expansion of the Universe.—Sir Arthur Eddington contributes a letter to the April number of the Observatory, in which he answers some questions submitted by Mr. B. M. Peek. The expansion is relative to our ordinary standards of length? for example, the wave-length of the red cadmium line may be taken as our standard. Secondly, the expansion is modified by gravitational action. Where there is periodic motion, as in the planetary system, the only effect of the expanding tendency is to lengthen the period corresponding to a given distance from the centre. This would be the case also in the rotation of the galaxy. He differs from Prof, de Sitter, who thinks that the expansion would be effective even in these cases. It is only when we come to the large scale of intergalactic distances that the expansive tendency prevails over the attractions of the galaxies on each other and periodic motion is no longer possible. He says that the problem may also be treated on the basis of taking the radius of space as constant, and our standards of length and time as shrinking. This assumption leads to the anomaly that an infinite number of terrestrial years would have a finite sum, and all matter would disappear in a finite time. Thus he prefers the “expansion of the universe” to the “contraction of the atom“.