AN explanation is due to our readers for the unusually large proportion of this week's issue of NATURE devoted to “Letters to the Editor”. In NATURE of February 10, we published an enlarged paper to provide accommodation for twenty columns of correspondence; since then, we have printed a dozen or so letters each week, which have occupied altogether a hundred columns of space. In fairness to our correspondents, it should be said that many of them have acted upon our suggestion that communications should be reduced in length, but still it has been difficult to ensure that prompt publication of current work which is now so widely recognised as one of the chief functions of our correspondence columns. In the circumstances, it has been decided once more to publish an extra number of pages of correspondence, in order to reduce the waiting list, and the present issue of NATURE therefore contains thirty-two columns under the heading “Letters to the Editor”. Of the twenty-nine communications printed, about a half are* from centres in Great Britain and Ireland. The remainder come from places so widespread as Copenhagen, Leningrad, Moscow and Warsaw in Europe, Boulder, Chicago, Harvard and Montreal in North America, Sendai in Japan, Cairo, and Kyancutta (South Australia). They provide further evidence, if such be needed, of the wide circulation of this journal and the keen activity with which scientific problems are being attacked in many parts of the world.